Memory device and electronic apparatus including the same

ABSTRACT

The inventive concept provides a memory device, in which memory cells are arranged to have a low variation in electrical characteristics and thereby enhanced reliability, an electronic apparatus including the memory device, and a method of manufacturing the memory device. In the memory device, memory cells at different levels may be covered with spacers having different thicknesses, and this may control resistance characteristics (e.g., set resistance) of the memory cells and to reduce a vertical variation in electrical characteristics of the memory cells. Furthermore, by adjusting the thicknesses of the spacers, a sensing margin of the memory cells may increase.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119 of KoreanPatent Application No. 10-2016-0010078, filed on Jan. 27, 2016, in theKorean Intellectual Property Office, the disclosure of which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The inventive concept relates to a memory device and a method offabricating the same, and more particularly, to a cross-point stackmemory device and a method of fabricating the same.

DISCUSSION OF RELATED ART

To meet an increasing demand for small and lightweight electronicproducts, highly-integrated semiconductor devices are generallyrequired. To this end, a three-dimensional (3D) cross-point stack memorydevice in which a memory cell is located at an intersection of twointersecting electrodes has been proposed. The proposed memory devicemay provide high-density data storage with minimal cell size. However,due to the increasing need for the downscaling of the cross-point stackmemory device, it may need to further reduce the dimension of each layerin the memory device. In this case, to obtain the desired reliability ofthe memory device, it may need to control variations in electricalcharacteristics of the memory cells.

SUMMARY

The inventive concept provides a memory device configured to reducevariations in electrical characteristics of memory cells and therebyenhance reliability, an electronic apparatus including the memorydevice, and a method of manufacturing the memory device.

According to an aspect of the inventive concept, there is provided amemory device including a first electrode line layer provided on asubstrate, the first electrode line layer including a plurality of firstelectrode lines which extend in a first direction and are spaced apartfrom each other, a second electrode line layer provided on the firstelectrode line layer, the second electrode line layer including aplurality of second electrode lines which extend in a second directiondifferent from the first direction and are spaced apart from each other,a third electrode line layer provided on the second electrode linelayer, the third electrode line layer including a plurality of firstelectrode lines, a first memory cell layer provided between the firstand second electrode line layers, the first memory cell layer includinga plurality of first memory cells arranged at respective intersectionsof the plurality of first electrode lines of the first electrode linelayer and the plurality of second electrode lines, a second memory celllayer provided between the second and third electrode line layers, thesecond memory cell layer including a plurality of second memory cellsarranged at respective intersections of the plurality of secondelectrode lines and the plurality of first electrode lines of the thirdelectrode line layer, and spacers including first spacers covering sidesurfaces of the plurality of first memory cells and second spacerscovering side surfaces of the plurality of second memory cells. Each ofthe plurality of first and second memory cells may include a selectiondevice, an intermediate electrode, and a variable resistance patternstacked in an upward or downward direction, and the first spacers mayhave a thickness different from that of the second spacers.

According to another aspect of the inventive concept, there is provideda memory device including a first electrode line layer provided on asubstrate, the first electrode line layer including a plurality of firstelectrode lines which extend in a first direction and are spaced apartfrom each other, a second electrode line layer provided on the firstelectrode line layer, the second electrode line layer including aplurality of second electrode lines which extend in a second directiondifferent from the first direction and are spaced apart from each other,a third electrode line layer provided on the second electrode linelayer, the third electrode line layer including a plurality of firstelectrode lines, a first memory cell layer provided between the firstand second electrode line layers, the first memory cell layer includinga plurality of first memory cells arranged at respective intersectionsof the plurality of first electrode lines of the first electrode linelayer and the plurality of second electrode lines, a second memory celllayer provided between the second and third electrode line layers, thesecond memory cell layer including a plurality of second memory cellsarranged at respective intersections of the plurality of secondelectrode lines and the plurality of first electrode lines of the thirdelectrode line layer, and spacers including first spacers covering sidesurfaces of the plurality of first memory cells and second spacerscovering side surfaces of the plurality of second memory cells. Each ofthe plurality of first and second memory cells may include a selectiondevice, an intermediate electrode, and a variable resistance patternstacked in an upward or downward direction, and at least one of theplurality of first spacers and the plurality of second spacers mayinclude a material exerting a compressive or tensile stress on thevariable resistance patterns.

According to still another aspect of the inventive concept, there isprovided an electronic apparatus including a processor configured toperform commands and to process data, a memory channel including atleast one signal line connected to the processor, a first memory deviceconnected to the processor through the memory channel, the first memorydevice including a first level memory having a first operation speed anda nonvolatile property, and a second memory device connected to theprocessor through the memory channel, the second memory device includinga second level memory having a second operation speed that is fasterthan the first operation speed. The first level memory may include atleast two memory cell layers, each of which has a cross-point structureand includes a plurality of memory cells and spacers respectivelycovering side surfaces of the plurality of memory cells. Each of theplurality of memory cells may include a selection device, anintermediate electrode, and a variable resistance pattern. The spacerscovering the side surfaces of the plurality of memory cells in one ofthe at least two memory cell layers may have a thickness different fromthat of the spacers covering the side surfaces of the plurality ofmemory cells in at least one other of the at least two memory celllayers.

According to still another aspect of the inventive concept, there isprovided a method of manufacturing a memory device, the methodincluding: forming a first electrode line layer on a substrate, thefirst electrode line layer including a plurality of first electrodelines extending in a first direction and spaced apart from each other ina second direction different from the first direction; forming a firstmemory cell layer on the first electrode line layer, the first memorycell including a plurality of first memory cells, of which each includesa first lower electrode, a first selection device, a first intermediateelectrode, a first heating electrode, and a first variable resistancepattern sequentially stacked, the plurality of first memory cellselectrically connected to the plurality of first electrode lines andspaced apart from each other in the first and second directions; forminga first inner spacer on side surfaces of the first lower electrode andthe first selection device for each of the plurality of first memorycells; forming a first spacer on side surfaces of the first innerspacer, the first immediate electrode, the first heating electrode, andthe first variable resistance pattern for each of the plurality of firstmemory cells; forming a second electrode line layer on the first memorycell layer, the second electrode line layer including a plurality ofsecond electrode lines extending in the second direction, spaced apartfrom each other in the first direction, and electrically connected tothe plurality of first memory cells; forming a second memory cell layeron the second electrode line layer, the second memory cell including aplurality of second memory cells, of which each includes a second lowerelectrode, a second selection device, a second intermediate electrode, asecond heating electrode, and a second variable resistance patternsequentially stacked, the plurality of second memory cells electricallyconnected to the plurality of second electrode lines and spaced apartfrom each other in the first and second directions; forming a secondinner spacer on side surfaces of the second lower electrode and thesecond selection device for each of the plurality of second memorycells; forming a second spacer on at least side surfaces of the secondimmediate electrode, the second heating electrode, and the secondvariable resistance pattern for each of the plurality of second memorycells; and forming a third electrode line layer on the second memorycell layer, the third electrode line layer including a plurality ofthird electrode lines extending in the first direction, spaced apartfrom each other in the second direction, and electrically connected tothe plurality of second memory cells, in which the first spacer may havea thickness different from that of the second spacer.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the inventive concept will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is an equivalent circuit diagram of a memory device according toan exemplary embodiment of the inventive concept;

FIG. 2 is a perspective view of a memory device according to anexemplary embodiment of the inventive concept;

FIG. 3 is a sectional view taken along lines X-X′ and Y-Y′ of FIG. 2;

FIG. 4 is a graph showing variations in set and reset resistances (Rsetand Rreset) of a memory cell, caused by a variation in thickness of aspacer according to an exemplary embodiment of the inventive concept;

FIG. 5 is a diagram schematically illustrating ion diffusion paths,which are formed in a variable resistance pattern by a voltage appliedto a memory cell according to an exemplary embodiment of the inventiveconcept;

FIG. 6 is a graph schematically showing a voltage-current curve of aselection device exhibiting an ovonic threshold switching (OTS)property;

FIGS. 7 to 14, 15A, and 15B are sectional views of memory devicesaccording to an exemplary embodiment of the inventive concept andcorresponding to the sectional view of FIG. 3;

FIG. 16 is a perspective view of a memory device according to anexemplary embodiment of the inventive concept;

FIG. 17 is a sectional view taken along lines 2X-2X′ and 2Y-2Y′ of FIG.16;

FIG. 18 is a perspective view of a memory device according to anexemplary embodiment of the inventive concept;

FIG. 19 is a sectional view taken along lines 3X-3X′ and 3Y-3Y′ of FIG.18;

FIG. 20 is a sectional view of a memory device according to an exemplaryembodiment of the inventive concept and corresponding to the sectionalview of FIG. 17;

FIG. 21 is a sectional view of a memory device according to an exemplaryembodiment of the inventive concept and corresponding to the sectionalview of FIG. 19;

FIGS. 22A to 22L are sectional views for describing a process offabricating a memory device (e.g., of FIG. 3) according to an exemplaryembodiment of the inventive concept;

FIGS. 23A to 23C are sectional views for describing a process offabricating a memory device (e.g., of FIG. 3) according to an exemplaryembodiment of the inventive concept; and

FIG. 24 is a block diagram of a computer system according to anexemplary embodiment of the inventive concept.

Since the drawings in FIGS. 1-24 are intended for illustrative purposes,the elements in the drawings are not necessarily drawn to scale. Forexample, some of the elements may be enlarged or exaggerated for claritypurpose.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the inventive concept are shown.

FIG. 1 is an equivalent circuit diagram of a memory device 100 accordingto an exemplary embodiment of the inventive concept.

Referring to FIG. 1, the memory device 100 may include lower word linesWL11 and WL12, which extend in a first direction X and are spaced apartfrom each other in a second direction Y perpendicular to the firstdirection X, and upper word lines WL21 and WL22, which extend in thefirst direction X and are spaced apart from each other in the seconddirection Y and spaced apart from the lower word lines WL11 and WL12 ina third direction Z perpendicular to the first and second directions Xand Y. In addition, the memory device 100 may include common bit linesBL1, BL2, BL3, and BL4, which are spaced apart from each other in thefirst direction X and spaced apart from the upper word lines WL21 andWL22 and the lower word lines WL11 and WL12 in the third direction Z,and are extending in the second direction Y.

First and second memory cells MC1 and MC2 may be provided, respectively,between the common bit lines BL1, BL2, BL3, and BL4 and the lower wordlines WL11 and WL12, and between the common bit lines BL1, BL2, BL3, andBL4 and the upper word lines WL21 and WL22. For example, the firstmemory cells MC1 may be arranged at respective intersections of thecommon bit lines BL1, BL2, BL3, and BL4 and the lower word lines WL11and WL12, and each of the first memory cells MC1 may include a variableresistance pattern ME for storing data and a selection device SW forselecting the variable resistance pattern ME. The second memory cellsMC2 may be arranged at respective intersections of the common bit linesBL1, BL2, BL3, and BL4 and the upper word lines WL21 and WL22, and eachof the second memory cells MC2 may also include the variable resistancepattern ME for storing data and the selection device SW for selectingthe variable resistance pattern ME. Meanwhile, the selection device SWmay be referred to as a switching device, isolation device, or an accessdevice. The selection device SW may be used to access the variableresistance pattern ME during programming or reading of the variableresistance pattern ME.

The first and second memory cells MC1 and MC2 may have substantially thesame structure and may be arranged in the third direction Z. Forexample, in the first memory cell MC1 arranged between the lower wordline WL11 and the common bit line BL1, the selection device SW may beelectrically connected to the lower word line WL11, the variableresistance pattern ME may be electrically connected to the common bitline BL1, and the variable resistance pattern ME and the selectiondevice SW may be connected in series to each other. Similarly, in thesecond memory cell MC2 arranged between the upper word line WL21 and thecommon bit line BL1, the variable resistance pattern ME may beelectrically connected to the upper word line WL21, the selection deviceSW may be electrically connected to the common bit line BL1, and thevariable resistance pattern ME and the selection device SW may beconnected in series to each other.

The inventive concept is not limited to the above example. For instance,unlike that shown in FIG. 1, in each of the first and second memorycells MC1 and MC2, positions of the selection device SW and the variableresistance pattern ME may be exchanged for each other. In addition, whenviewed along the third direction Z, the first and second memory cellsMC1 and MC2 may be arranged to have a symmetrical arrangement about acorresponding one of the common bit lines BL1, BL2, BL3, and BL4. Forexample, the first and second memory cells MC1 and MC2 may besymmetrically arranged about the common bit line BL1 in such a way thatthe variable resistance pattern ME and the selection device SW of thefirst memory cell MC1 may be connected to the lower word line WL11 andthe common bit line BL1, respectively, and the variable resistancepattern ME and the selection device SW of the second memory cell MC2 maybe connected to the upper word line WL21 and the common bit line BL1,respectively.

Hereinafter, a method of operating the memory device 100 will bedescribed briefly. For example, in the case where a voltage is appliedto the variable resistance pattern ME of the first memory cell MC1 orthe second memory cell MC2 through the word lines WL11, WL12, WL21, andWL22 and the common bit lines BL1, BL2, BL3, and BL4, a current may flowthrough the variable resistance pattern ME. The variable resistancepattern ME may include, for example, a phase-change material (PCM) layerwhich is reversibly switchable between a first state and a second state.However, the variable resistance pattern ME is not limited thereto, andany variable resistance material whose resistance can be changed by avoltage applied thereto may be used as the variable resistance patternME. For example, if at least one of the first and second memory cellsMC1 and MC2 is selected, resistance of the variable resistance patternME of the selected memory cell may be reversibly switched between afirst state and a second state by a voltage applied to the variableresistance pattern ME.

Digital data, such as “0” or “1”, may be stored in the first and secondmemory cells MC1 and MC2, depending on a change in resistance of thevariable resistance pattern ME. Similarly, with the resistance of thevariable resistance patterns ME changed back to their original values,the stored digital data may be erased from the first and second memorycells MC1 and MC2. For example, a high-resistance state “0” and alow-resistance state “1” may be written as data in the first and secondmemory cells MC1 and MC2. Here, an operation of changing ahigh-resistance state “0” into a low-resistance state “1” may bereferred to as a “set operation”, and an operation of changing alow-resistance state “1” into a high-resistance state “0” may bereferred to as a “reset operation”. However, digital data stored in thefirst and second memory cells MC1 and MC2 according to an exemplaryembodiment of the inventive concept are not limited to thehigh-resistance state “0” and the low-resistance state “1”, but variousresistance states may be stored in the first and second memory cells MC1and MC2. For example, by applying voltage of different amplitudes tocertain material, the electrical resistance may be switched to multiplevalues. These multiple resistance values instead of binary resistancestates may be used to store data.

The first and second memory cells MC1 and MC2 may be selectivelyaddressed by selecting the word lines WL11, WL12, WL21, and WL22 and thecommon bit lines BL1, BL2, BL3, and BL4, and the first and second memorycells MC1 and MC2 may be programmed by applying signals to the wordlines WL11, WL12, WL21, and WL22 and the common bit lines BL1, BL2, BL3,and BL4. Also, resistances (or programmed data) of the variableresistance patterns of the first and second memory cells MC1 and MC2 maybe determined by measuring currents flowing through the common bit linesBL1, BL2, BL3, and BL4.

FIG. 2 is a perspective view of a memory device according to anexemplary embodiment of the inventive concept, and FIG. 3 is a sectionalview taken along lines X-X′ and Y-Y′ of FIG. 2. To reduce complexity inthe drawings and to provide a better understanding of the inventiveconcept, insulating layers 160 a, 160 b, 160 c, 160 d, and 160 e areomitted from FIG. 3.

Referring to FIGS. 2 and 3, the memory device 100 may include asubstrate 101, a first electrode line layer 110L, a second electrodeline layer 120L, a third electrode line layer 130L, a first memory celllayer MCL1, a second memory cell layer MCL2, first spacers 150-1, andsecond spacers 150-2.

As shown in FIGS. 2 and 3, an interlayered insulating layer 105 may bearranged on the substrate 101. The interlayered insulating layer 105 maybe formed of an oxide material (e.g., silicon oxide) or a nitridematerial (e.g., silicon nitride), and may be used to electricallyseparate the first electrode line layer 110L from the substrate 101.Although, in the memory device 100 according to the present embodiment,the interlayered insulating layer 105 is arranged on the substrate 101,this is just an example of the present embodiment. For example, in thememory device 100 according to the present embodiment, an integratedcircuit layer may be arranged on the substrate 101, and memory cells maybe arranged on the integrated circuit layer. The integrated circuitlayer may include, for example, a peripheral circuit for operation ofthe memory cells and/or a core circuit for calculation. Here, thestructure, in which an integrated circuit layer including a peripheralcircuit and/or a core circuit is arranged on a substrate and memorycells are arranged on the integrated circuit layer, may be called a‘cell-on-peripheral (COP) structure’.

The first electrode line layer 110L may include a plurality of firstelectrode lines 110, which extend in a first direction X and areprovided to be parallel to each other and spaced apart from each otherin a second direction Y that is different from the first direction X.The second electrode line layer 120L may include a plurality of secondelectrode lines 120, which extend in the second direction Y and areprovided to be parallel to each other and spaced apart from each otherin the first direction X. In addition, the third electrode line layer130L may include a plurality of third electrode lines 130, which extendin the first direction X and are provided to be parallel to each otherand spaced apart from each other in the second direction Y. Although thethird electrode lines 130 are different from the first electrode lines110 in their positions in the third direction Z, the third electrodelines 130 may be substantially the same as the first electrode lines 110in terms of extension direction or arrangement. In this sense, the thirdelectrode lines 130 may be referred to as “the first electrode lines ofthe third electrode line layer 130L”.

In operational aspects of a memory device, the first and third electrodelines 110 and 130 may serve as word lines, and the second electrodelines 120 may serve as bit lines. Alternatively, the first and thirdelectrode lines 110 and 130 may serve as the bit lines, and the secondelectrode lines 120 may serve as the word lines. In the case where thefirst and third electrode lines 110 and 130 serve as the word lines, thefirst electrode lines 110 may serve as lower word lines and the thirdelectrode lines 130 may serve as upper word lines. In addition, thesecond electrode lines 120 may be shared by the lower word lines and theupper word lines. That is, the second electrode lines 120 may serve ascommon bit lines.

Each of the first electrode lines 110, the second electrode lines 120,and the third electrode lines 130 may include, for example, metals,conductive metal nitrides, conductive metal oxides, or combinationsthereof. For example, each of the first electrode lines 110, the secondelectrode lines 120, and the third electrode lines 130 may be formed ofor include tungsten (W), tungsten nitride (WN), gold (Au), silver (Ag),copper (Cu), aluminum (Al), titanium aluminum nitride (TiAlN), iridium(Ir), platinum (Pt), palladium (Pd), ruthenium (Ru), zirconium (Zr),rhodium (Rh), nickel (Ni), cobalt (Co), chromium (Cr), Tin (Sn), zinc(Zn), indium tin oxide (ITO), alloys thereof, or combinations thereof.Also, each of the first electrode lines 110, the second electrode lines120, and the third electrode lines 130 may include a metal layer and aconductive barrier layer covering at least a portion of the metal layer.The conductive barrier layer may be formed of or include, for example,Ti, TiN, Ta, TaN, or combinations thereof.

The first memory cell layer MCL1 may include a plurality of first memorycells 140-1, which are spaced apart from each other in the first andsecond directions X and Y and may serve as the first memory cells MC1 ofFIG. 1. The second memory cell layer MCL2 may include a plurality ofsecond memory cells 140-2, which are spaced apart from each other in thefirst and second directions X and Y and may serve as the second memorycells MC2 of FIG. 1. As shown in FIG. 2, the first electrode lines 110and the second electrode lines 120 may be provided to intersect eachother, and the second electrode lines 120 and the third electrode lines130 may be provided to intersect each other. The first memory cells140-1 may be provided between the first electrode line layer 110L andthe second electrode line layer 120L and at respective intersections ofthe first electrode lines 110 and the second electrode lines 120, andmay be connected to the first electrode lines 110 and the secondelectrode lines 120. The second memory cells 140-2 may be providedbetween the second and third electrode line layers 120L and 130L and atrespective intersections of the second and third electrode lines 120 and130, and may be connected to the second and third electrode lines 120and 130.

Each of the first and second memory cells 140-1 and 140-2 may beprovided to have a pillar-shaped structure with a rectangular section.Of course, the structures of the first and second memory cells 140-1 and140-2 are not limited thereto. For example, the first and second memorycells 140-1 and 140-2 may be provided to have various pillar structureswith circular, elliptical, and polygonal sections. Also, the first andsecond memory cells 140-1 and 140-2 may be provided to have a decreasingwidth in an upward or downward direction, depending on the method usedto form them. For example, in the case where the first and second memorycells 140-1 and 140-2 are formed via an etching process, the first andsecond memory cells 140-1 and 140-2 may be formed to have a decreasingwidth in an upward direction. In the case where the first and secondmemory cells 140-1 and 140-2 are formed via a damascene process, thefirst and second memory cells 140-1 and 140-2 may be formed to have adecreasing width in a downward direction. In the case where the etchingor damascene process is controlled to achieve a substantially verticalprofile, the vertical difference in width of each of the first andsecond memory cells 140-1 and 140-2 may be reduced or removed. Althoughthe first and second memory cells 140-1 and 140-2 are illustrated ashaving a vertical side surface profile so as to reduce complexity in thedrawings, the first and second memory cells 140-1 and 140-2 may beprovided to have a structure, whose bottom width is larger or smallerthan its top width.

Each of the first memory cells 140-1 and each of the second memory cells140-2 may include, respectively, a lower electrode 141-1 and a lowerelectrode 141-2, a selection device 143-1 and a selection device 143-2,an intermediate electrode 145-1 and an intermediate electrode 145-2, aheating electrode 147-1 and a heating electrode 147-2, and a variableresistance pattern 149-1 and a variable resistance pattern 149-2. Sincethe first and second memory cells 140-1 and 140-2 have substantially thesame structure, the following description will be given with referenceto the first memory cells 140-1, for convenience of description.

In an exemplary embodiment of the inventive concept, the variableresistance pattern 149-1 (or ME of FIG. 1) may include a phase-changematerial whose phase can be reversibly switched between amorphous andcrystalline states, depending on a heating time. In general,phase-change materials may exist in an amorphous and one or sometimesseveral crystalline phases, and they can be rapidly and repeatedlyswitched between these phases. For example, the variable resistancepattern 149-1 may include a material whose phase can be reversiblychanged using Joule's heat, which is generated when a voltage is appliedto both terminals of the variable resistance pattern 149-1, and whoseresistance can be changed by such a change in phase. In detail, thephase-change material may be in a high-resistance state when it has anamorphous phase and may be in a low-resistance state when it has acrystalline phase. In the memory device, the high- and low-resistancestates of the variable resistance pattern 149-1 may be defined andstored as “data 0” and “data 1”, respectively.

In an exemplary embodiment of the inventive concept, the variableresistance pattern 149-1 may include at least one of the elements ingroup VI of the periodic table (e.g., chalcogen elements) and optionallya chemical modifier containing at least one of the chemical elements ingroup III, IV or V. For example, the variable resistance pattern 149-1may be formed of or include Ge—Sb—Te (germanium-antimony-tellurium,GST). Here, in the above chemical formula, the hyphen (-) is used torepresent all chemical mixtures or compounds, in which listed elementsare contained. For example, the expression “Ge—Sb—Te” may representvarious materials such as Ge₂Sb₂Te₅, Ge₂Sb₂Te₇, Ge₁Sb₂Te₄, andGe₁Sb₄Te₇.

The variable resistance pattern 149-1 may include other variousphase-change materials, other than the above material (i.e., Ge—Sb—Te(GST)). Compositions of the phase-change materials for the variableresistance pattern 149-1 may contain mixtures of various elements whichinclude, but are not limited to: germanium (Ge), antimony (Sb),tellurium (Te), indium (In), selenium (Se), Gallium (Ga), arsenic (As),aluminum (Al), bismuth (Bi), Tin (Sn), oxygen (O), sulfur (S), nitrogen(N), gold (Au), palladium (Pd), titanium (Ti), cobalt (Co), silver (Ag),and nickel (Ni). For example, the variable resistance pattern 149-1 mayinclude at least one of Ge—Te, Sb—Te, In—Se, Ga—Sb, GeSb, In—Sb, As—Te,Al—Te, Bi—Sb—Te (BST), In—Sb—Te (IST), Ge—Sb—Te (GST), Te—Ge—As,Te—Sn—Se, Ge—Se—Ga, Bi—Se—Sb, Ga—Se—Te, Sn—Sb—Te, In—Sb—Ge, In—Ge—Te,Ge—Sn—Te, Ge—Bi—Te, Ge—Te—Se, As—Sb—Te, Sn—Sb—Bi, Ge—Te—O, GeSbTeN,Te—Ge—Sb—S, Te—Ge—Sn—O, Te—Ge—Sn—Au, Pd—Te—Ge—Sn, In—Se—Ti—Co,Ge—Sb—Te—Pd, Ge—Sb—Te—Co, GeBiSbTe, Sb—Te—Bi—Se, Ag—In—Sb—Te,Ge—Sb—Se—Te, Ge—Sn—Sb—Te, Ge—Te—Sn—Ni, Ge—Te—Sn—Pd, Ge—Te—Sn—Pt,GeInSbTe, In—Sn—Sb—Te, As—Ge—Sb—Te, and a combination thereof.

The variable resistance pattern 149-1 may be formed to have variouschemical stoichiometric compositions. A crystallization temperature, amelting temperature, and a crystallization energy of the variableresistance pattern 149-1 may vary depending on the chemicalstoichiometric composition, and thus, by changing the chemicalstoichiometric composition, a phase-change speed and a data retentionproperty of the variable resistance pattern 149-1 may be controlled.

The variable resistance pattern 149-1 may further include, for example,at least one of carbon (C), nitrogen (N), silicon (Si), oxygen (O),bismuth (Bi), boron (B), indium (In), and tin (Sn), which are used as animpurity. A driving current of the memory device 100 may be changed dueto the presence of the impurity. In addition, the variable resistancepattern 149-1 may further include a metallic element. For example, thevariable resistance pattern 149-1 may include at least one of aluminum(Al), gallium (Ga), zinc (Zn), titanium (Ti), chromium (Cr), manganese(Mn), iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo), ruthenium(Ru), palladium (Pd), hafnium (Hf), lanthanum (LA), tantalum (Ta),iridium (Ir), platinum (Pt), zirconium (Zr), thallium (Tl), lead (Pb),and polonium (Po). The presence of the metallic element may lead to anincrease in electric and thermal conductivities of the variableresistance pattern 149-1, and thus, the crystallization speed and theset speed of the variable resistance pattern 149-1 may increase. Also,in the case where the metallic element is contained in the variableresistance pattern 149-1, the data retention property of the variableresistance pattern 149-1 may be enhanced.

The variable resistance pattern 149-1 may have a multi-layeredstructure, in which two or more layers having different physicalproperties are stacked. The number or thicknesses of the layers of thevariable resistance pattern 149-1 may be variously changed. A barrierlayer may be further provided between the layers of the variableresistance pattern 149-1. The barrier layer may be configured to preventa material from being diffused between the layers of the variableresistance pattern 149-1. For example, the barrier layer may prevent amaterial from being diffused from a previously-formed layer of thelayers into a later-formed layer of the layers. The barrier layer mayinclude, but is not limited to: SiN, TiN, Ta₂O₅, WN, TaN, TiSiN, TaSiN,highly nitrogen doped GST, or a combination thereof.

The variable resistance pattern 149-1 may be provided to include aplurality of layers, which are alternatingly stacked on each other andcontain different materials from each other, to thereby have a superlattice structure. For example, the variable resistance pattern 149-1may include first and second layers, which are formed of Ge—Te andSb—Te, respectively, and are alternatingly stacked on each other.However, the materials for the first and second layers are not limitedto Ge—Te and Sb—Te, and the afore-described various materials may beused for the first and second layers.

As described above, the variable resistance pattern 149-1 may includethe phase-change material, but the inventive concept is not limitedthereto. For example, the variable resistance pattern 149-1 of thememory device 100 according to the present embodiment may includevarious other materials exhibiting the variable resistance property.

In an exemplary embodiment of the inventive concept, the variableresistance pattern 149-1 may contain a transition metal oxide, and inthis case, the memory device 100 may be a resistive random access memory(ReRAM). In the case where the variable resistance pattern 149-1contains the transition metal oxide, a programming operation may beperformed to create or destroy at least one electric path in thevariable resistance pattern 149-1. The variable resistance pattern 149-1may have a low resistance when the electric path is created and may havea high resistance when the electric path is destroyed. Such a differencein resistance level of the variable resistance pattern 149-1 may be usedto store data in the memory device 100.

In the case where the variable resistance pattern 149-1 is formed of thetransition metal oxide, the transition metal oxide may include at leastone of metallic elements (e.g., Ta, Zr, Ti, Hf, Mn, yttrium (Y), Ni, Co,Zn, niobium (Nb), Cu, Fe, and Cr). For example, the transition metaloxide may include one or more layers, each of which is formed of atleast one of Ta₂O_(5-x), TiO_(2-x), HfO_(2-x), MnO_(2-x), Y₂O_(3-x),NiO_(1-y), Nb₂O_(5-x), CuO_(1-y), and Fe₂O_(3-x). In the materialslisted above, the unknown parameters (i.e., x and y) may be selected tosatisfy the condition of 0≦x≦1.5 and 0≦y≦0.5, but the inventive conceptis not limited thereto.

In an exemplary embodiment of the inventive concept, the variableresistance pattern 149-1 may have a magnetic tunnel junction (MTJ)structure, in which two magnetic electrodes and a dielectric materialinterposed therebetween are provided, and in this case, the memorydevice 100 may be a magnetic RAM (MRAM).

The two magnetic electrodes described above may serve as a magnetizationfixed layer and a magnetization free layer, and the dielectric materialinterposed therebetween may serve as a tunnel barrier layer. Themagnetization fixed layer may have a fixed magnetization direction, andthe magnetization free layer may have a magnetization direction that canbe switched to be parallel or antiparallel to that of the magnetizationfixed layer. The magnetization directions of the magnetization fixedlayer and the magnetization free layer may be parallel to a surface ofthe tunnel barrier layer, but the inventive concept is not limitedthereto. For example, the magnetization fixed layer and themagnetization free layer may have magnetization directions that areperpendicular to a surface of the tunnel barrier layer.

In the case where the magnetization directions of the magnetization freelayer and the magnetization fixed layer are parallel to each other, thevariable resistance pattern 149-1 may have a first resistance. Bycomparison, in the case where the magnetization directions of themagnetization free layer and the magnetization fixed layer areantiparallel to each other, the variable resistance pattern 149-1 mayhave a second resistance. Such a difference in resistance level of thevariable resistance pattern 149-1 may be used to store data in thememory device 100. The magnetization direction of the magnetization freelayer may be changed using a spin torque of electrons in a programcurrent.

Each of the magnetization fixed layer and the magnetization free layermay include a magnetic material. Here, the magnetization fixed layer mayfurther include an anti-ferromagnetic material, allowing a ferromagneticmaterial in the magnetization fixed layer to have a fixed magnetizationdirection. The tunnel barrier layer may be formed of or include at leastone of oxide materials, each of which contains, for example, one of Mg,Ti, Al, MgZn, and MgB, but the inventive concept is not limited thereto.Examples of ferromagnetic material may include, but are not limited to:Fe, Ni, Co, and many of their alloys. Examples of antiferromagnetic mayinclude, but are not limited to: MnO, FeO, CoO, NiO, Cr, Mn, MnO₄, MnS,FeCl₃, and MnF₂.

The selection device 143-1 (or SW of FIG. 1) may serve as a currentadjustment layer controlling a flow of current passing therethrough. Theselection device 143-1 may include a layer whose resistance can bechanged by a voltage applied to both sides of the selection device143-1. For example, the selection device 143-2 may include an ovonicthreshold switching (OTS) material exhibiting an OTS property. Withregard to the function of the selection device 143-1 including the OTSmaterial, when a voltage lower than a threshold voltage V_(T) is appliedto the selection device 143-1, the selection device 143-1 may be in ahigh-resistance state preventing a current from flowing therethrough,and when a voltage higher than the threshold voltage V_(T) is applied tothe selection device 143-1, the selection device 143-1 may be in alow-resistance state, allowing a current to flow therethrough. Also, inthe case where a current flowing through the selection device 143-1 issmaller than a holding current, the selection device 143-1 may beswitched to the high-resistance state. The OTS property of the selectiondevice 143-1 will be described in more detail with reference to FIG. 6.

The selection device 143-1 may include a chalcogenide material servingas the OTS material. The OTS materials and phase-change materials (PCM)may be in the same class, but the OTS materials are usually frozen inthe amorphous phase. In an exemplary embodiment of the inventiveconcept, the chalcogenide material may include at least one of theelements in group VI of the periodic table (e.g., chalcogen elements)and optionally a chemical modifier containing at least one of thechemical elements in group III, IV or V. Sulfur (S), selenium (Se), andtellurium (Te) may be typical chalcogen elements, which may be includedin the selection device 143-1. The presence of a divalent bond and lonepair electrons may be regarded as characteristic features of thechalcogen elements. In the chalcogenide materials, chalcogen elementsmay be bonded with each other through divalent bonding to form a chainstructure and a cyclic structure, and the lone pair electrons may serveas an electron source for forming a conductive filament. For example,trivalent or tetravalent modifiers (e.g., aluminum (Al), gallium (Ga),indium (In), germanium (Ge), tin (Sn), silicon (Si), phosphorus (P),arsenic (As), and antimony (Sb)) may be contained in the chain andcyclic structures of the chalcogen elements to adjust structuralrigidity of the chalcogenide material, and the chalcogenide material maybe classified into a switching material and a phase-change material,based on its crystallization or another structural rearrangementability.

In an exemplary embodiment of the inventive concept, the selectiondevice 143-1 may contain silicon (Si), tellurium (Te), arsenic (As),germanium (Ge), indium (In), or any combination thereof. For example,compounds of the selection device 143-1 may contain about 14 at. %silicon (Si), about 39 at. % tellurium (Te), about 37 at. % arsenic(As), about 9 at. % germanium (Ge), and about 1 at. % indium (In). Here,the atomic percent “at. %” may be given by the percentage of one kind ofatom relative to the total number of atoms, and this term will be usedhaving the same meaning below.

In an exemplary embodiment of the inventive concept, the selectiondevice 143-1 may include silicon (Si), tellurium (Te), arsenic (As),germanium (Ge), sulfur (S), selenium (Se), or combinations thereof. Forexample, the compounds of the selection device 143-1 may contain about 5at. % silicon (Si), about 34 at. % tellurium (Te), about 28 at. %arsenic (As), about 11 at. % germanium (Ge), about 21 at. % sulfur (S),and about 1 at. % selenium (Se).

In an exemplary embodiment of the inventive concept, the selectiondevice 143-1 may include tellurium (Te), arsenic (As), germanium (Ge),sulfur (S), selenium (Se), antimony (Sb), or combinations thereof. Forexample, the compositions for the selection device 143-1 may containabout 21 at. % tellurium (Te), about 10 at. % arsenic (As), about 15 at.% germanium (Ge), about 2 at. % sulfur (S), about 50 at. % selenium(Se), and about 2 at. % antimony (Sb).

In the memory device 100 according to an exemplary embodiment of theinventive concept, the selection device 143-1 is not limited to the OTSmaterial. For example, the selection device 143-1 may include variousmaterials capable of providing a switching function or a deviceselection function. As an example, the selection device 143-1 mayinclude, for example, a diode, a tunnel junction, a PNP diode, a bipolarjunction transistor (BJT), or a mixed ionic-electronic conduction (MIEC)device.

The heating electrode 147-1 may be arranged between the intermediateelectrode 145-1 and the variable resistance pattern 149-1. The heatingelectrode 147-1 may be used to heat the variable resistance pattern149-1 in the set or reset operation. The heating electrode 147-1 may beformed of or include a conductive material, which does not react withthe variable resistance pattern 149-1 and generate a sufficient amountof heat to change a phase of the variable resistance pattern 149-1. Inan exemplary embodiment of the inventive concept, the heating electrode147-1 may be formed of or include, for example, TiN, TiSiN, TiAlN,TaSiN, TaAlN, TaN, WSi, WN, TiW, MoN, NbN, TiBN, ZrSiN, WSiN, WBN,ZrAlN, MoAlN, TiAl, TiON, TiAlON, WON, TaON, carbon (C), silicon carbide(SiC), silicon carbon nitride (SiCN), carbon nitride (CN), titaniumcarbon nitride (TiCN), tantalum carbon nitride (TaCN), high meltingpoint metals containing combinations thereof, or nitrides thereof.However, the material for the heating electrode 147-1 is not limitedthereto.

In an exemplary embodiment of the inventive concept, the heatingelectrode 147-1 may be formed of carbon-based materials which include,but are not limited to: amorphous carbon (C), graphene, graphite, carbonnanotube (CNT), amorphous diamond-like carbon (DLC), silicon carbide(SiC), boron carbide (BC), silicon carbon nitride (SiCN), carbon nitride(CN), titanium carbon nitride (TiCN), tantalum carbon nitride (TaCN),and other similar carbon-based materials.

The lower electrode 141-1 and the intermediate electrode 145-1 may be incontact with the selection device 143-1, and may serve as a current pathand may be formed of a conductive material. For example, each of thelower electrode 141-1 and the intermediate electrode 145-1 may be formedof or include, for example, a metal, a conductive metal nitride, aconductive metal oxide, or combinations thereof. As an example, thelower electrode 141-1 and the intermediate electrode 145-1 may include aTiN layer, but the inventive concept is not limited thereto. In anexemplary embodiment of the inventive concept, each of the lowerelectrode 141-1 and the intermediate electrode 145-1 may include aconductive layer, which is formed of metals or conductive metalnitrides, and at least one conductive barrier layer, which is formed tocover at least a portion of the conductive layer. The conductive barrierlayer may be formed of or include, for example, a metal oxide, a metalnitride, or combinations thereof, but the inventive concept is notlimited thereto.

In general, when the selection device 143-1 is based on the OTSproperty, the selection device 143-1 may include a chalcogenide materialthat is in an amorphous state. However, the down-scaling of the memorydevice 100 may lead to a reduction in thickness, width, and distance ofthe variable resistance pattern 149-1, the selection device 143-1, theheating electrode 147-1, the lower electrode 141-1 and/or theintermediate electrode 145-1. Accordingly, when a phase of the variableresistance pattern 149-1 is changed by heat generated in the heatingelectrode 147-1 during operations of the memory device 100, theselection device 143-1 adjacent to the heating electrode 147-1 may beaffected by the heating process. For example, the selection device 143-1may be partially crystallized by heat generated in the heating electrode147-1 adjacent thereto. That is, there may be deterioration or damage ofthe selection device 143-1.

By comparison, in the memory device 100 according to the presentembodiment, the intermediate electrode 145-1 may be thickly formed toprevent heat generated in the heating electrode 147-1 from beingtransferred to the selection device 143-1. For such thermal insulation,the intermediate electrode 145-1 may be formed to be thicker than thelower electrode 141-1, as shown in FIGS. 2 and 3. For example, theintermediate electrode 145-1 may have a thickness in a range of about 10nm to about 100 nm. However, the thickness of the intermediate electrode145-1 is not limited to this range. Also, for the thermal insulation,the intermediate electrode 145-1 may include at least one thermalbarrier layer. The thickness of the barrier layer may be in a range ofabout 1 nm to about 50 nm. In the case where the intermediate electrode145-1 includes two or more thermal barrier layers, the intermediateelectrode 145-1 may have a structure, in which the thermal barrierlayers and electrode layers are alternatingly stacked.

The first spacers 150-1 may be provided to enclose side surfaces of thefirst memory cells 140-1. The second spacers 150-2 may be provided toenclose side surfaces of the second memory cells 140-2. Since the firstand second spacers 150-1 and 150-2 are provided to enclose the sidesurfaces of the first and second memory cells 140-1 and 140-2, the firstand second spacers 150-1 and 150-2 may be used to protect the first andsecond memory cells 140-1 and 140-2 (in particular, the variableresistance patterns 149-1 and 149-2 and/or the selection devices 143-1and 143-2). For example, the first and second spacers 150-1 and 150-2may prevent the first and second memory cells 140-1 and 140-2 from beingunnecessarily contaminated or etched in a subsequent process (e.g., in acleaning process or a metal patterning process). For example, the firstand second spacers 150-1 and 150-2 may have etch resistance toward theetchant used in the subsequent process.

The first and second spacers 150-1 and 150-2 may exert a tensile orcompressive stress on the first and second memory cells 140-1 and 140-2,thereby enhancing the current characteristics of the first and secondmemory cells 140-1 and 140-2. The tensile or compressive stress exertedon the first and second memory cells 140-1 and 140-2 from the first andsecond spacers 150-1 and 150-2 will be described in more detail withreference to FIGS. 18 to 21.

The first and second spacers 150-1 and 150-2 may be formed of orinclude, for example, at least one material of, oxides (e.g., siliconoxide (SiO₂) or aluminum oxide (Al₂O₃)), nitrides (e.g., silicon nitride(Si₃N₄)), and oxynitride (e.g., silicon oxynitride), which are capableof protecting the first and second memory cells 140-1 and 140-2. Thefirst and second spacers 150-1 and 150-2 may be formed using at leastone conformal deposition method (e.g., thermal and plasma chemical vapordeposition (CVD) or atomic layer deposition (ALD) techniques).

In the memory device 100 according to the present embodiment, the firstspacer 150-1 may have a first thickness T1, and the second spacer 150-2may have a second thickness T2. As shown in FIGS. 2 and 3, the firstthickness T1 of the first spacer 150-1 may be greater than the secondthickness T2 of the second spacer 150-2. Here, the thickness of each ofthe first and second spacers 150-1 and 150-2 may be defined as athickness measured in a direction normal to the side surface of each ofthe first and second memory cells 140-1 and 140-2. For example, thethickness of each of the first and second spacers 150-1 and 150-2 may bedefined as a thickness of a portion that is substantially parallel tothe side surfaces of the first and second memory cells 140-1 and 140-2.In addition, the first thickness T1 of the first spacer 150-1 may be athickness measured in a direction normal to a side surface of thevariable resistance pattern 149-1 of the first memory cell 140-1, andthe second thickness T2 of the second spacer 150-2 may be a thicknessmeasured in a direction normal to a side surface of the variableresistance pattern 149-2 of the second memory cell 140-2. That is, thethicknesses are measure at the locations that the spacers are coveringthe variable resistance patterns.

In the memory device 100 according to the present embodiment, by thicklyforming the first spacers 150-1 of the first memory cells 140-1 andthinly forming the second spacers 150-2 of the second memory cells140-2, resistance characteristics of the first and second memory cells140-1 and 140-2 may be enhanced. This may reduce the variation inelectrical characteristics between the first and second memory cells140-1 and 140-2, which are provided at different levels or layers in thememory device 100 according to the present embodiment.

In the case where the memory device has a three-dimensional (3D)cross-point stack structure, memory cells in the memory device maysuffer from an increase in variation in electrical characteristics and areduction in sensing margin (S/M). In detail, when compared with theconventional single-layer structure, the 3D stack structure may have anincreased memory cell density and an increased vertical variation incharacteristics of memory cells, which are responsible for the increasedvariation in electrical characteristics of the memory cells found in the3D stack structure. The vertical variation in memory cellcharacteristics will be described in more detail with reference to FIG.5.

The reduction of the S/M may result from the down-scaling of the memorydevice. That is, the down-scaling of the memory device may lead to anincrease in the set resistance, without any substantial change in thereset resistance, and consequently a reduction in ratio between the setand reset resistances (i.e., in the sensing margin). In general, the setresistance of the memory device may be low (e.g., of order kΩ), and thereset resistance may be very high (e.g., of order MΩ). Given thatdecreasing of an area causes an increase in electric resistance, thedown-scaling of the memory device (i.e. decreasing an area in the memorydevice) may lead to an increase in the set resistance, but there may besubstantially no change in the reset resistance. Simply, in the casewhere a current path is formed of a conductor, an amount of a currentmay vary depending on a sectional area of the current path, whereas, inthe case where the current path is formed of an insulator, there may beno current flowing through the current path, regardless of its sectionalarea.

For the 3D cross-point stack structure of the memory device, it may needto control a variation in electrical characteristics of memory cells andit may also need to increase or maximize the S/M of the memory device.In the memory device 100 according to the present embodiment, sincespacers covering memory cells at different levels are formed to havedifferent thicknesses, they may increase or maximize an S/M of a memorydevice and may reduce a variation in electrical characteristics of thememory cells. For example, in the case where the first and secondspacers 150-1 and 150-2 are formed to have substantially the samethickness, a set resistance of the first memory cells 140-1 of the firstmemory cell layer MCL1 may be higher than that of the second memorycells 140-2 of the second memory cell layer MCL2. In this case, if thefirst spacers 150-1 of the first memory cells 140-1 are thickly formedand the second spacers 150-2 of the second memory cells 140-2 are thinlyformed, a difference in set resistance between the first and secondmemory cells 140-1 and 140-2 may be reduced or removed. In the oppositecase, a difference in set resistance between the first and second memorycells 140-1 and 140-2 may be reduced or removed by thinly forming thefirst spacers 150-1 of the first memory cells 140-1 and by thicklyforming the second spacers 150-2 of the second memory cells 140-2.

Based on the reason described above, the adjustment of the spacerthickness may be used to control the set resistance of the memory cells.However, for the same reason as described with reference to thedown-scaling, the adjustment of the spacer thickness may result in aminute change in reset resistance of the memory cells. Accordingly, theadjustment of the spacer thickness may affect a variation in electricalcharacteristics of memory cells or an S/M property through a change inset resistance of memory cells. Changes in set and reset resistancescaused by a change of a spacer thickness will be described in moredetail with reference to FIG. 4.

The memory device 100 according to an exemplary embodiment of theinventive concept may further include a first inner spacer 152-1 and asecond inner spacer 152-2. The first inner spacer 152-1 may be providedto cover the lower electrode 141-1 and the selection device 143-1 of thefirst memory cell 140-1, and the second inner spacer 152-2 may beprovided to cover the lower electrode 141-2 and the selection device143-2 of the second memory cell 140-2. The first and second innerspacers 152-1 and 152-2 may be formed, using a process separated fromthat for the first and second spacers 150-1 and 150-2, for moreeffective protection of the selection devices 143-1 and 143-2. However,in an exemplary embodiment of the inventive concept, the first andsecond inner spacers 152-1 and 152-2 may be omitted.

As shown in FIGS. 2 and 3, the first inner spacer 152-1 may be coveredwith the first spacer 150-1, whereas the second inner spacer 152-2 maynot be covered with the second spacer 150-2. However, in an exemplaryembodiment of the inventive concept, the second inner spacer 152-2 mayalso be covered with the second spacer 150-2. Furthermore, although thefirst and second inner spacers 152-1 and 152-2 are illustrated to havesubstantially the same structure and substantially the same thickness,they may be formed to have different structures and/or differentthicknesses.

As described above, thicknesses of the first and second spacers 150-1and 150-2 may be adjusted to control a variation in electricalcharacteristics (e.g., set resistance) of memory cells, but thisadjustment or control may be mainly related to resistancecharacteristics of the variable resistance patterns 149-1 and 149-2. Inother words, this means that the controlling of the thicknesses of thefirst and second spacers 150-1 and 150-2 may be used to change acrystalline state of each of the variable resistance patterns 149-1 and149-2 or to control a resistance corresponding to such a crystallinestate (i.e., set and reset resistances corresponding to the crystallineand amorphous states, respectively).

In the memory device 100 according to the present embodiment, thecontrol of variation in electrical characteristics through theadjustment of the spacer thickness is not limited to the control of theresistance of the variable resistance patterns 149-1 and 149-2. Forexample, the adjustment of the spacer thickness may be used to controlcurrent characteristics of the selection devices 143-1 and 143-2.Meanwhile, since, unlike the variable resistance patterns 149-1 and149-2, the selection devices 143-1 and 143-2 do not have the phasechangeable material, the control of the current characteristics of theselection devices 143-1 and 143-2 may mean the control of thresholdvoltages of the selection devices 143-1 and 143-2.

The current characteristics of the selection devices 143-1 and 143-2 maybe controlled by adjusting the thicknesses of the first spacer 150-1 andthe second spacer 150-2. However, the current characteristics of theselection devices 143-1 and 143-2 may be controlled by adjustingthicknesses of the first and second spacers 150-1 and 150-2 and/or thefirst and second inner spacers 152-1 and 152-2.

As shown in FIG. 2, a first insulating layer 160 a may be arrangedbetween the first electrode lines 110, and a second insulating layer 160b may be arranged between the first memory cells 140-1 of the firstmemory cell layer MCL1. Also, a third insulating layer 160 c may bearranged between the second electrode lines 120, a fourth insulatinglayer 160 d may be arranged between the second memory cells 140-2 of thesecond memory cell layer MCL2, and a fifth insulating layer 160 e may bearranged between the third electrode lines 130. The first to fifthinsulating layers 160 a-160 e may be formed of the same insulatingmaterial, but in an exemplary embodiment of the inventive concept, atleast one of the first to fifth insulating layers 160 a-160 e may beformed of an insulating material different from those of the others. Thefirst to fifth insulating layers 160 a-160 e may be formed of adielectric material (e.g., oxide or nitride) and may separate devices ineach layer electrically from each other. Meanwhile, air gaps may beformed instead of at least one of the second and fourth insulatinglayers 160 b and 160 d. In the case where the air gaps are formed, aninsulating liner having a specific thickness may be formed between theair gaps and the first memory cells 140-1 and/or between the air gapsand the second memory cells 140-2.

In the memory device 100 according to the present embodiment, since thespacers 150-1 and 150-2 of the first and second memory cells 140-1 and140-2 at different levels or layers are formed to have differentthicknesses, resistance (e.g., set resistance) of the first and secondmemory cells 140-1 and 140-2 may be controlled. Accordingly, byadjusting the thicknesses of the spacers 150-1 and 150-2 of the firstand second memory cells 140-1 and 140-2, a vertical variation inresistance characteristics of the first and second memory cells 140-1and 140-2 of the memory device 100 according to the present embodimentmay be reduced. In addition, by adjusting the thicknesses of the spacers150-1 and 150-2 of the first and second memory cells 140-1 and 140-2, anS/M of the first and second memory cells 140-1 and 140-2 of the memorydevice 100 according to the present embodiment may increase.Accordingly, by adjusting the thicknesses of the spacers 150-1 and150-2, a memory device having a 3D cross-point stack structure (e.g., ahigh integration density) and enhanced reliability may be realized.

FIG. 4 is a graph showing variations in set and reset resistances (Rsetand Rreset) of a memory cell, caused by a variation in thickness of aspacer (or a spacer thickness). In FIG. 4, the x-axis represents thespacer thickness, and the left- and right-hand y-axes represent the setresistance (●) and the reset resistance (▪). The spacer thickness andthe set and reset resistances are relative values, which are given inarbitrary units (a.u.) to allow for a relative comparison therebetween.

Referring to FIG. 4, as the spacer thickness increased, both the set andreset resistances decreased. Here, as described above, the spacerthickness is defined as a thickness measured in a direction normal tothe side surface of each of the first and second memory cells 140-1 and140-2. As shown, the increase in the spacer thickness led to adifference in reduction rate between the set and reset resistances. Forexample, the increase in the spacer thickness led to a large reductionof the set resistance but a small reduction of the reset resistance.This result may be obtained for the same reason as that for the changeof the resistance associated with a down-scaling of a memory device. Inother words, when the memory cell has the set resistance (i.e., arelatively low resistance), the resistance of the memory cell may beeasily changed by a change in area, structure, or any externalenvironment. By comparison, when the memory cell has the resetresistance (i.e., a very high resistance), the resistance of the memorycell may be scarcely affected by a change in area, structure, or anyexternal environment.

As described above, the S/M is defined as a ratio between the setresistance and the reset resistance, it is obvious that, if the resetresistance does not change and the set resistance increases, the S/Mwill decrease. For example, in the graph of FIG. 4, the S/M decreases ina left direction (i.e., toward Tmin) and increases in a right direction(i.e., toward Tmax). Accordingly, in the present embodiment, bycontrolling the spacer thickness of the memory device 100 and the ratiobetween the set and reset resistances, the S/M may be increased ormaximized. Furthermore, such an increase of the S/M may greatly enhancethe reliability of the memory device 100.

In the memory device 100 according to the present embodiment, thespacers 150-1 and 150-2 having different thicknesses may be formed tocover the first and second memory cells 140-1 and 140-2 provided atdifferent levels or layers, and such a difference in thickness betweenthe spacers 150-1 and 150-2 may be used to control the resistance (inparticular, the set resistance) of the first and second memory cells140-1 and 140-2 at different levels or layers. Accordingly, in thememory device 100 according to the present embodiment, by controllingthe thicknesses of the spacers 150-1 and 150-2, the S/M of the first andsecond memory cells 140-1 and 140-2 may increase, and consequently, amemory device with enhanced reliability may be realized.

FIG. 5 is a diagram schematically illustrating ion diffusion paths,which are formed in a variable resistance pattern by a voltage appliedto a memory cell.

Referring to FIG. 5, a first memory cell 50A may include a firstelectrode 20A, a variable resistance pattern 30A, and a second electrode40A, which are sequentially stacked. The first electrode 20A may includea conductive material capable of generating a sufficient amount of heat,which is required to change a phase of the variable resistance pattern30A, and may serve as the heating electrodes 147-1 and 147-2 of FIGS. 2and 3. In the case where a positive voltage is applied to the firstelectrode 20A and a negative voltage is applied to the second electrode40A, an electric current flowing from the first electrode 20A to thesecond electrode 40A through the variable resistance pattern 30A may beproduced in the memory cell 50A, as depicted by an arrow C_A.

In the case where a current flows through the first electrode 20A, heatmay be generated in the first electrode 20A, and thus, a change in phaseof the variable resistance pattern 30A may start to occur at a portion30A_P, which is adjacent to an interface between the first electrode 20Aand the variable resistance pattern 30A. For example, in a “reset”operation, the portion 30A_P of the variable resistance pattern 30A maybe changed from a crystalline state (i.e., the low-resistance state) toan amorphous state (i.e., the high-resistance state), and thus, theremay be a difference in diffusion speed between cations and anions in theportion 30A_P, when a voltage is applied thereto. In more detail,cations (e.g., antimony ions (Sb⁺)) in the portion 30A_P of the variableresistance pattern 30A may have a diffusion speed higher than that ofanions (e.g., tellurium ions (Te⁻)). This may lead to an increase in anamount of antimony ions (Sb⁺) diffusing toward the second electrode 40A,to which the negative voltage is applied. For example, a diffusion speedof antimony ions (Sb+) in a direction toward the second electrode 40Amay be higher than that of tellurium ions (Te⁻) in a direction towardthe first electrode 20A.

By comparison, a second memory cell 50B may include a first electrode20B, a variable resistance pattern 30B, and a second electrode 40B, andnegative and positive voltages are respectively applied to the first andsecond electrodes 20B and 40B to generate a current flowing from thesecond electrode 40B to the first electrode 20B through the variableresistance pattern 30B, as depicted by an arrow C_B.

In the case where a current flows through the first electrode 20B, heatmay be generated in the first electrode 20B, and thus, a change in phaseof the variable resistance pattern 30B may start to occur at a portion30B_P, which is adjacent to an interface between the first electrode 20Band the variable resistance pattern 30B. Here, in the portion 30B_P ofthe variable resistance pattern 30B, a diffusion speed of antimony ions(Sb⁺) may be faster than that of tellurium ions (Te⁻), and thus, theremay be an increase in an amount of antimony ions (Sb⁺) diffusing towardthe first electrode 20B, to which a negative voltage is applied.

In the case of the second memory cell 50B, antimony ions (Sb⁺) may havean increased concentration near an interface between the first electrode20B and the variable resistance pattern 30B. For example, there may be alocal variation in concentration of the variable resistance pattern 30B.By comparison, in the case of the first memory cell 50A, tellurium ions(Te⁻) may have an increased concentration near an interface between thefirst electrode 20A and the variable resistance pattern 30A. Forexample, there may be a local variation in concentration of the variableresistance pattern 30A.

An ion or vacancy distribution in the variable resistance patterns 30Aand 30B may vary, depending on a magnitude of a voltage to be applied tothe variable resistance patterns 30A and 30B, a direction of a currentflowing through the variable resistance patterns 30A and 30B, andgeometry of the variable resistance patterns 30A and 30B and the firstelectrodes 20A and 20B. That is, a concentration of ions or vacanciesmay be locally changed in the variable resistance patterns 30A and 30B,and thus, even when the variable resistance patterns 30A and 30B areapplied with substantially the same voltage, there may be a variation inresistance of the variable resistance patterns 30A and 30B and thememory cells 50A and 50B may exhibit different operationalcharacteristics (e.g., different resistance characteristics).

Although the ion diffusion paths for antimony ions (Sb⁺) and telluriumions (Te⁻) have been briefly described with reference to FIG. 5, theinventive concept is not limited thereto. In particular, similar to thevariable resistance patterns 149-1 of the first memory cells 140-1described with reference to FIGS. 2 and 3, the variable resistancepatterns 30A and 30B may be doped, for example, with at least one oftellurium (Te), selenium (Se), germanium (Ge), antimony (Sb), bismuth(Bi), lead (Pb), tin (Sn), indium (In), silver (Ag), arsenic (As),sulfur (S), phosphorus (P), and mixtures thereof, and may be doped withimpurities such as, for example, nitrogen (N), oxygen (O), silicon (Si),carbon (C), boron (B), dysprosium (Dy), or combinations thereof.Accordingly, ion diffusion in the variable resistance patterns 30A and30B may vary depending on a kind and composition of a material containedin the variable resistance patterns 30A and 30B and a kind andconcentration of impurities, and thus, a variation in operationalcharacteristics of the memory cells 50A and 50B may increase.

Referring back to FIGS. 2 and 3, a plurality of the first memory cells140-1 and a plurality of the second memory cells 140-2 may be verticallyspaced apart from each other with respect to the second electrode lines120. In the case where a positive reset voltage Vreset is applied to thesecond electrode lines 120 and a ground voltage is applied to the firstand third electrode lines 110 and 130, substantially the same voltage(i.e., the reset voltage Vreset) may be applied to the first memorycells 140-1 and the second memory cell 140-2. However, the first memorycells 140-1 and the second memory cells 140-2 may be provided below andon the second electrode lines 120, and thus, as described above, aportion of the variable resistance pattern 149-1, which is positionedadjacent to an interface between the variable resistance patterns 149-1and the heating electrodes 147-1 of the first memory cells 140-1, maydiffer from a portion of the variable resistance pattern 149-2, which ispositioned adjacent to an interface between the variable resistancepatterns 149-2 and the heating electrodes 147-2 of the second memorycells 140-2, in terms of ion distribution or concentration distribution.Accordingly, the first and second memory cells 140-1 and 140-2 may havedifferent operational characteristics (e.g., different resistancecharacteristics) from each other.

In the memory device 100 according to the present embodiment, since thespacers 150-1 and 150-2 are formed on the side surfaces of the first andsecond memory cells 140-1 and 140-2, they may prevent the variableresistance patterns 149-1 and 149-2 and/or the selection devices 143-1and 143-2 from deteriorating or being contaminated or damaged.Furthermore, since the spacers 150-1 and 150-2 covering the first andsecond memory cells 140-1 and 140-2 at different levels or layers areformed to have different thicknesses, a vertical variation in resistanceor current characteristics of the first and second memory cells 140-1and 140-2 at the different levels or layers may be reduced or minimized.

Since the memory device 100 according to the present embodiment includesthe selection devices 143-1 and 143-2 including the OTS material, aprocess for forming a transistor or a diode may be omitted. For example,in the case where a diode is provided, it is necessary to perform a hightemperature thermal treatment process for activating impuritiescontained in the diode, but the high temperature thermal treatmentprocess may lead to damage or contamination of the variable resistancepatterns 149-1 and 149-2 containing the phase-change material. Bycomparison, in the case of the memory device 100 according to thepresent embodiment, it is unnecessary to perform complex processes forforming a transistor or a diode, and thus, it may prevent the variableresistance patterns 149-1 and 149-2 from being damaged or contaminatedby the complex processes. Accordingly, the use of the memory device 100according to the present embodiment may realize a highly-reliablesemiconductor device.

In the case where a transistor or a diode is provided as a selectiondevice, it is necessary to provide the transistor or the diode in asubstrate, and this may lead to a difficulty in realizing a stack-typememory device including a plurality of vertically-stacked layers. Inparticular, since there is a risk that the variable resistance patterns149-1 and 149-2 may be damaged or contaminated by the high temperaturethermal treatment for activating the diode, it may be very difficult torealize a cross-point stack structure, in which the diodes are providedon the variable resistance patterns 149-1 and 149-2. However, in thecase where the selection devices 143-1 and 143-2 with the OTS propertyare used instead of the diodes, the memory device 100 according to thepresent embodiment can be realized in the form of a 3D cross-point stackstructure including a plurality of vertically-stacked layers.Accordingly, the memory device 100 can have a greatly increasedintegration density.

FIG. 6 is a graph schematically showing a voltage-current behavior of aselection device exhibiting an OTS property.

Referring to FIG. 6, a first curve 61 shows a voltage-current behaviorof a selection device, when there is substantially no current passingthrough the selection device. Here, the selection device may serve as aswitching device having a threshold voltage V_(T) (or a first voltagelevel 63). The current flowing through the selection device may besubstantially negligible, while a voltage applied to the selectiondevice gradually increases from a state of zero voltage and zero currentto the threshold voltage V_(T) (i.e., the first voltage level 63).However, if the applied voltage exceeds the threshold voltage V_(T), thecurrent passing through the selection device may abruptly increase, andthe applied voltage may drop to a saturation voltage V_(S) (or a secondvoltage level 64).

A second curve 62 shows a voltage-current behavior of the selectiondevice, when there is a current passing through the selection device. Ifthe current passing through the selection device exceeds a first currentlevel 66, the voltage applied to the selection device may increase up toa level that is slightly higher than the second voltage level 64. Forexample, even if the current passing through the selection deviceincreases from the first current level 66 to a considerably higher level(e.g., a second current level 67), the voltage applied to the selectiondevice may slightly increase from the second voltage level 64. That is,once the current starts to flow through the selection device, thevoltage applied to the selection device may be maintained around thesaturation voltage V_(S) (i.e., the second voltage level 64). If thecurrent is reduced to a holding current level (i.e., the first currentlevel 66) or lower, the selection device may be switched again to aresistive state, and thus, the current may be effectively blocked untilthe voltage is increased to the threshold voltage V_(T).

FIGS. 7 to 14, 15A, and 15B are sectional views of memory devicesaccording to an exemplary embodiment of the inventive concept andcorresponding to the sectional view of FIG. 3. In the description thatfollows, descriptions of features identical to those of FIGS. 2 to 3will be kept to a minimum or omitted in order to avoid redundancy.

Referring to FIG. 7, a memory device 100 a according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3, in that the inner spacer is not provided. Forexample, in the memory device 100 of FIG. 3, the first inner spacer152-1 may be provided to enclose the lower electrode 141-1 and theselection device 143-1 of the first memory cells 140-1, and the secondinner spacer 152-2 may be provided to enclose the lower electrode 141-2and the selection device 143-2 of the second memory cells 140-2.

By comparison, the first and second inner spacers may not be provided inthe memory device 100 a according to the present embodiment.Accordingly, the lower electrode 141-1 and the selection device 143-1 ofthe first memory cell 140-1, along with the intermediate electrode145-1, the heating electrode 147-1, and the variable resistance pattern149-1, may be enclosed by a first spacer 150 a-1, and the lowerelectrode 141-2 and the selection device 143-2 of the second memory cell140-2, along with the intermediate electrode 145-2, the heatingelectrode 147-2, and the variable resistance pattern 149-2, may beenclosed by a second spacer 150 a-2.

In the memory device 100 a according to the present embodiment, thefirst spacer 150 a-1 may be thickly formed, and the second spacer 150a-2 may be thinly formed. Accordingly, a reduction in resistance of thefirst memory cells 140-1 may be larger than that of the second memorycells 140-2. In other words, in the case where the spacer is not formedor is formed to have the same thickness, the resistance of the firstmemory cells 140-1 may be higher than that of the second memory cells140-2. In such a case, like the memory device 100 a according to thepresent embodiment, by thickly forming the spacer 150 a-1 of the firstmemory cells 140-1 and thinly forming the spacers 150 a-2 of the secondmemory cells 140-2, a difference in resistance between the first andsecond memory cells 140-1 and 140-2 may be reduced, and consequently thevertical variation in resistance characteristics of the first and secondmemory cells 140-1 and 140-2 at different levels or layers may bereduced. In the opposite case, the first spacer 150 a-1 may be thinlyformed, and the second spacer 150 a-2 may be thickly formed. Thus, thethickness of the first spacer 150 a-1 or the second spacer 150 a-2 maybe adjusted so that the first and second memory cells 140-1 and 140-2may have substantially the same resistance. By controlling thethicknesses of the spacers 150 a-1 and 150 a-2, the S/M of the first andsecond memory cells 140-1 and 140-2 may increase and consequently, amemory device with enhanced reliability may be realized. Alternatively,or in addition, at least one of the first spacer 150 a-1 and the secondspacer 150 a-2 may independently comprise one of a material exerting acompressive stress and a material exerting a tensile stress on thecorresponding variable resistance pattern 149-1 and 149-2, and thecompressive or tensile stress property of the first spacer 150 a-1 orthe second spacer 150 a-2 may be adjusted so that the first and secondmemory cells 140-1 and 140-2 may have substantially the same resistance.Accordingly, the memory device 100 a may have enhanced reliability. Theeffect of tensile or compressive stress exerted on the memory cell toreduce or increase resistance will be described in more detail withreference to FIGS. 18 to 21.

In the memory device 100 a according to the present embodiment, thefirst and second inner spacers may not be provided, and thus, when thereis a need to control current characteristics (e.g., threshold voltage)of the selection devices 143-1 and 143-2, a method of adjustingthicknesses of the first and second spacers 150 a-1 and 150 a-2 may beused to enhance the current characteristics of the selection devices143-1 and 143-2.

Referring to FIG. 8, a memory device 100 b according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3, in that a first spacer 150 b-1 is formed to have amulti-layered structure. For example, in the case of the memory device100 of FIG. 3, the first spacer 150-1 may be formed of a single layer.By comparison, in the memory device 100 b according to the presentembodiment, the first spacer 150 b-1 may have a double-layer structureincluding an inner layer 151 and an outer layer 153. Since the firstspacer 150 b-1 has the multi-layered structure, electric characteristicsof the first memory cells 140-1 (in particular, of the variableresistance pattern 149-1 and/or the selection device 143-1) may be moreprecisely controlled. In other words, by variously changing a materialproperty of each of the layers constituting the first spacer 150 b-1,resistance characteristics of the first memory cells 140-1 may bevariously controlled. For example, even when the first spacers 150 b-1are formed to have the same thickness, the resistance characteristics ofthe first memory cells 140-1 may be changed, depending on whether theinner layer 151 is formed of a tensile stress material or a compressivestress material.

In the memory device 100 b according to an exemplary embodiment of theinventive concept, the first spacer 150 b-1 may have a multi-layeredstructure including three or more layers. Furthermore, the second spacer150-2 may also have a multi-layered structure including two or morelayers. However, like the memory device 100 of FIG. 3, the first spacer150 b-1 may be formed to be thicker than the second spacer 150-2. Toreduce the vertical variation in resistance characteristics of the firstand second memory cells 140-1 and 140-2, it may require to properlyselect a tensile stress material or a compressive stress material forthe inner layer 151, and to adjust the thickness of the first spacer 150b-1 or the second spacer 150-2, so that the first and second memorycells 140-1 and 140-2 may have substantially the same resistance.Accordingly, the memory device 100 b may have enhanced reliability. Inthe case where the first and second spacers 150 b-1 and 150-2 are formedto have the multi-layered structure, a conformal deposition technique(e.g., ALD) may be used to form the first and second spacers 150 b-1 and150-2.

Referring to FIG. 9, a memory device 100 c according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3 in terms of positions of the heating electrodes147-1 and 147-2 of the first and second memory cells 140 a-1 and 140a-2. For example, in the memory device 100 of FIG. 3, the heatingelectrodes 147-1 and 147-2 may be arranged between the intermediateelectrodes 145-1 and 145-2 and the variable resistance patterns 149-1and 149-2. By comparison, in the memory device 100 c according to thepresent embodiment, the heating electrode 147-1 of the first memory cell140 a-1 may be arranged between the variable resistance pattern 149-1and the second electrode line 120, and the heating electrode 147-2 ofthe second memory cell 140 a-2 may be arranged between the variableresistance pattern 149-2 and the third electrode line 130.

Since, as described above, the heating electrodes 147-1 and 147-2 may beused to heat the variable resistance patterns 149-1 and 149-2, theheating electrodes 147-1 and 147-2 may be provided at positions,allowing them to be in contact with the variable resistance patterns149-1 and 149-2, respectively. Accordingly, the heating electrodes 147-1and 147-2 may be provided on or under the variable resistance patterns149-1 and 149-2, respectively. Meanwhile, the heating electrodes 147-1and 147-2 may be provided on the variable resistance patterns 149-1 and149-2, respectively, and this may suppress or prevent heat of theheating electrodes 147-1 and 147-2 from being transferred to theselection devices 143-1 and 143-2. Also, in the case where the heatingelectrodes 147-1 and 147-2 are provided on the variable resistancepatterns 149-1 and 149-2, they may prevent the variable resistancepatterns 149-1 and 149-2 from being contaminated or etched when apatterning process is performed to form the third electrode lines 130.Similar to the memory device 100 of FIG. 3, in the memory device 100 c,the thickness of the first spacer 150-1 or the second spacer 150-2 maybe adjusted so that the first and second memory cells 140 a-1 and 140a-2 may have substantially the same resistance. Alternatively, or inaddition, at least one of the first spacer 150-1 and the second spacer150-2 may independently comprise one of a material exerting acompressive stress and a material exerting a tensile stress on thecorresponding variable resistance pattern 149-1 and 149-2, and thecompressive or tensile stress property of the first spacer 150-1 or thesecond spacer 150-2 may be adjusted so that the first and second memorycells 140 a-1 and 140 a-2 may have substantially the same resistance.Accordingly, the memory device 100 c may have enhanced reliability.

Referring to FIG. 10, a memory device 100 d according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3, in that the upper electrodes 148-1 and 148-2 areadditionally formed. For example, in the memory device 100 of FIG. 3,the variable resistance patterns 149-1 of the first memory cells 140-1may be directly connected to the second electrode lines 120, and thevariable resistance patterns 149-2 of the second memory cells 140-2 maybe directly connected to the third electrode lines 130. By comparison,the memory device 100 d according to the present embodiment may includefirst memory cells 140 b-1, which include the upper electrodes 148-1provided between the variable resistance pattern 149-1 and the secondelectrode lines 120, and second memory cells 140 b-2, which include theupper electrodes 148-2 provided between the variable resistance pattern149-2 and the third electrode lines 130.

The upper electrodes 148-1 and 148-2 may serve as current paths, likethe lower and intermediate electrodes 141-1, 141-2, 145-1, and 145-2.Also, the upper electrodes 148-1 and 148-2 may prevent the variableresistance patterns 149-1 and 149-2 from being contaminated or etchedwhen a patterning process is performed to form the third electrode lines130. Furthermore, the upper electrodes 148-1 and 148-2 may prevent acontact failure between the variable resistance patterns 149-1 and 149-2and the third electrode lines 130. The upper electrodes 148-1 and 148-2may be formed of the same conductive material as that of the lower andintermediate electrodes 141-1, 141-2, 145-1, and 145-2. Similar to thememory device 100 of FIG. 3, in the memory device 100 d, the thicknessof the first spacer 150-1 or the second spacer 150-2 may be adjusted sothat the first and second memory cells 140 b-1 and 140 b-2 may havesubstantially the same resistance. Alternatively, or in addition, atleast one of the first spacer 150-1 and the second spacer 150-2 mayindependently comprise one of a material exerting a compressive stressand a material exerting a tensile stress on the corresponding variableresistance pattern 149-1 and 149-2, and the compressive or tensilestress property of the first spacer 150-1 or the second spacer 150-2 maybe adjusted so that the first and second memory cells 140 b-1 and 140b-2 may have substantially the same resistance. Accordingly, the memorydevice 100 d may have enhanced reliability.

Referring to FIG. 11, a memory device 100 e according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3, in that the memory device 100 e may includevariable resistance patterns 149′-1 and 149′-2 that are formed to benarrower than other patterns. For example, in the memory device 100 ofFIG. 3, the variable resistance patterns 149-1 and 149-2 may be formedto have a width substantially the same as those of other patterns (e.g.,the heating electrodes 147-1 and 147-2). By comparison, in the memorydevice 100 e according to the present embodiment, the variableresistance patterns 149′-1 and 149′-2 may be formed to have a smallerwidth than those of other patterns (e.g., the heating electrodes 147-1and 147-2).

Meanwhile, given that the memory cells 140 c-1 and 140 c-2 of the memorydevice 100 e have a pillar shape, a horizontal sectional area of each ofthe variable resistance patterns 149′-1 and 149′-2 may be smaller thanthat of each of the other patterns (e.g., the heating electrodes 147-1and 147-2).

Reliability of the memory device 100 e may be strongly dependent on astructure or size of the variable resistance pattern 149′-1 or 149′-2.Thus, the variable resistance patterns 149′-1 and 149′-2 may be formedusing a different method from that for the other patterns and thus mayhave a different size from those of the other patterns. Of course, thevariable resistance patterns 149′-1 and 149′-2 may be formed to have alarger size than those of the other patterns. Similar to the memorydevice 100 of FIG. 3, in the memory device 100 e, the thickness of thefirst spacer 150-1 or the second spacer 150-2 may be adjusted so thatthe first and second memory cells 140 c-1 and 140 c-2 may havesubstantially the same resistance. Alternatively, or in addition, atleast one of the first spacer 150-1 and the second spacer 150-2 mayindependently comprise one of a material exerting a compressive stressand a material exerting a tensile stress on the corresponding variableresistance pattern 149′-1 and 149′-2, and the compressive or tensilestress property of the first spacer 150-1 or the second spacer 150-2 maybe adjusted so that the first and second memory cells 140 c-1 and 140c-2 may have substantially the same resistance. Accordingly, the memorydevice 100 e may have enhanced reliability.

Although not shown, the selection devices 143-1 and 143-2 may have adifferent size from those of the other patterns.

Referring to FIG. 12, a memory device 100 f according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3, in that the memory device 100 f may include lowerelectrodes 141′-1 and 141′-2 and selection devices 143′-1 and 143′-2that are formed in the damascene structure. As described above, thememory cells 140 d-1 and 140 d-2 may be formed via an etching process ora damascene process.

The etching process may include sequentially forming layers constitutingthe memory cells and then etching the layers using a mask pattern toform patterns constituting the memory cells. In the case where theetching process is used to form the memory cells, each of the memorycells may be formed to have a narrow upper portion and a wide lowerportion. By comparison, the damascene process may include forming aninsulating layer, patterning the insulating layer using a mask patternto form trenches in the insulating layer, and then, filling the trencheswith layers constituting the memory cells. In the case where thedamascene process is used to form the memory cells, each of the memorycells may be formed to have a wide upper portion and a narrow lowerportion. However, in the case of the damascene process, it may bedifficult to sequentially form a plurality of layers in the trench, andthus, the damascene process may be generally applied to one or twolayers, and other layers may be patterned using the etching process.

In the memory device 100 f according to the present embodiment, thelower electrodes 141′-1 and 141′-2 and the selection devices 143′-1 and143′-2 may be formed via the damascene process, and the intermediateelectrodes 145-1 and 145-2, the heating electrodes 147-1 and 147-2 andthe variable resistance patterns 149-1 and 149-2 provided thereon may beformed via the etching process. Accordingly, the lower electrodes 141′-1and 141′-2 and the selection devices 143′-1 and 143′-2 may be formed tohave a downward decreasing width.

By precisely controlling an etching step in the damascene process, theside surface of the memory cells 140 d-1 and 140 d-2 may be formed to besubstantially perpendicular to the top surface of the substrate 101. Inthis case, the upper and lower portions of each of the lower electrodes141′-1 and 141′-2 and the selection devices 143′-1 and 143′-2 may havesubstantially the same width. Meanwhile, in order to clearly show thatthe lower electrodes 141′-1 and 14′1-2 and the selection devices 143′-1and 143′-2 are formed via the damascene process, the slope of the sidesurface of them is exaggeratedly illustrated in FIG. 12.

In the memory device 100 f according to the present embodiment, sincethe lower electrodes 141′-1 and 141′-2 and the selection devices 143′-1and 143′-2 are formed via the damascene process, the spacers 150 c-1 and150 c-2 may be formed on only the side surfaces of the intermediateelectrodes 145-1 and 145-2, the heating electrodes 147-1 and 147-2, andthe variable resistance patterns 149-1 and 149-2. That is, although notshown, in the case where the lower electrodes 141′-1 and 141′-2 and theselection devices 143′-1 and 143′-2 are formed via the damasceneprocess, the spacers 150 c-1 and 150 c-2 may not be formed on the sidesurfaces of the lower electrodes 141′-1 and 141′-2 and the selectiondevices 143′-1 and 143′-2, which are covered with a previously-formedinsulating layer. In the present embodiment, the thickness of the firstspacer 150 c-1 or the second spacer 150 c-2 may be adjusted so that thefirst and second memory cells 140 d-1 and 140 d-2 may have substantiallythe same resistance. Alternatively, or in addition, at least one of thefirst spacer 150 c-1 and the second spacer 150 c-2 may independentlycomprise one of a material exerting a compressive stress and a materialexerting a tensile stress on the corresponding variable resistancepattern 149-1 and 149-2, and the compressive or tensile stress propertyof the first spacer 150 c-1 or the second spacer 150 c-2 may be adjustedso that the first and second memory cells 140 d-1 and 140 d-2 may havesubstantially the same resistance. Accordingly, the memory device 100 fmay have enhanced reliability.

Referring to FIG. 13, a memory device 100 g according to an exemplaryembodiment of the inventive concept may be similar to the memory device100 f of FIG. 12, in that the lower electrodes 141′-1 and 141′-2 and theselection devices 143′-1 and 143′-2 are formed in the damascenestructure. However, in the memory device 100 g according to the presentembodiment, lower spacers 152 a-1 and 152 a-2 may be formed on the sidesurfaces of the lower electrodes 141′-1 and 141′-2 and the selectiondevices 143′-1 and 143′-2.

In the case of the memory device 100 g according to the presentembodiment, when the lower electrodes 141′-1 and 141′-2 and theselection devices 143′-1 and 143′-2 are formed via a damascene process,a spacer may be formed on a side surface of a trench, and then, thelower electrodes 141′-1 and 141′-2 and the selection devices 143′-1 and143′-2 may be formed in the trench provided with the spacer.Accordingly, the memory device 100 g according to the present embodimentmay include the lower spacers 152 a-1 and 152 a-2, which are formed onthe side surfaces of the lower electrodes 141′-1 and 141′-2 and theselection devices 143′-1 and 143′-2.

In the memory device 100 g according to the present embodiment, bycontrolling thicknesses of the lower spacers 152 a-1 and 152 a-2,current characteristics (e.g., a threshold voltage) of the selectiondevices 143′-1 and 143′-2 may be controlled. Of course, resistancecharacteristics of the memory cells 140 d-1 and 140 d-2 or the variableresistance patterns 149-1 and 149-2 may be controlled by adjusting thethicknesses of the spacers 150 c-1 and 150 c-2. For example, thethickness of the first spacer 150 c-1 or the second spacer 150 c-2 maybe adjusted so that the first and second memory cells 140 d-1 and 140d-2 may have substantially the same resistance. Alternatively, or inaddition, at least one of the first spacer 150 c-1 and the second spacer150 c-2 may independently comprise one of a material exerting acompressive stress and a material exerting a tensile stress on thecorresponding variable resistance pattern 149-1 and 149-2, and thecompressive or tensile stress property of the first spacer 150 c-1 orthe second spacer 150 c-2 may be adjusted so that the first and secondmemory cells 140 d-1 and 140 d-2 may have substantially the sameresistance. Accordingly, the memory device 100 g may have enhancedreliability.

Referring to FIG. 14, a memory device 100 h according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 of FIG. 3, in that the variable resistance patterns 149″-1and 149″-2 are formed in the damascene structure. In more detail, in thememory device 100 h according to the present embodiment, the lowerelectrodes 141-1 and 141-2, the selection devices 143-1 and 143-2, theintermediate electrodes 145-1 and 145-2, and the heating electrodes147-1 and 147-2 may be formed via an etching process, and the variableresistance patterns 149″-1 and 149″-2 may be formed via a damasceneprocess. In addition, the inner spacers 152-1 and 152-2 may be formed onthe side surfaces of the lower electrodes 141-1 and 141-2 and theselection devices 143-1 and 143-2. In an exemplary embodiment of theinventive concept, the inner spacers 152-1 and 152-2 may be omitted.

In the memory device 100 h according to the present embodiment, upperspacers 155-1 and 155-2 may be formed on the side surfaces of thevariable resistance patterns 149″-1 and 149″-2. The upper spacers 155-1and 155-2 may be formed by using the same method as that for the lowerspacers 152 a-1 and 152 a-2 of the memory device 100 g of FIG. 13. Forexample, trenches may be formed on an insulating layer, and then, theupper spacers 155-1 and 155-2 may be formed on side surfaces of thetrenches. In this case, the variable resistance patterns 149″-1 and149″-2 may be formed to fill remaining spaces of the trenches providedwith the upper spacers 155-1 and 155-2. In an exemplary embodiment ofthe inventive concept, the upper spacers 155-1 and 155-2 may be omitted.

The thicknesses of the upper spacers 155-1 and 155-2 may be adjusted toenhance resistance characteristics of the variable resistance patterns149″-1 and 149″-2. For example, the thickness of the upper spacer 155-1or the upper spacer 155-2 may be adjusted so that the first and secondmemory cells 140 e-1 and 140 e-2 may have substantially the sameresistance. Alternatively, or in addition, at least one of the firstspacer 155-1 and the second spacer 155-2 may independently comprise oneof a material exerting a compressive stress and a material exerting atensile stress on the corresponding variable resistance pattern 149″-1and 149″-2, and the compressive or tensile stress property of the firstspacer 155-1 or the second spacer 155-2 may be adjusted so that thefirst and second memory cells 140 e-1 and 140 e-2 may have substantiallythe same resistance. Accordingly, the memory device 100 h may haveenhanced reliability. Also, the thicknesses of the inner spacers 152-1and 152-2 may be adjusted to enhance the current characteristics of theselection devices 143-1 and 143-2. That is, in the memory device 100 haccording to the present embodiment, the electric characteristics of thevariable resistance patterns 149″-1 and 149″-2 and the selection devices143-1 and 143-2 can be independently controlled.

To reduce complexity in the drawings, the side surfaces of the upperspacers 155-1 and 155-2 are illustrated in FIG. 14 to be almostperpendicular to the top surface of the substrate 101. However, in anexemplary embodiment of the inventive concept, the side surfaces of theupper spacers 155-1 and 155-2 may be formed to be slightly inclined atan angle to the top surface of the substrate 101. For example, in thecase where the damascene process is used to form the upper spacers 155-1and 155-2 and the variable resistance patterns 149″-1 and 149″-2, theupper spacers 155-1 and 155-2 may be formed to have a decreasing widthin a downward direction.

Referring to FIG. 15A, a memory device 100 i according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 h of FIG. 14, in that the memory device 100 i may includevariable resistance patterns 149 a-1 and 149 a-2 that are formed in thedamascene structure but have an “L”-shaped structure. In more detail, inthe memory device 100 i according to the present embodiment, the lowerelectrodes 141-1 and 141-2, the selection devices 143-1 and 143-2, theintermediate electrodes 145-1 and 145-2, and the heating electrodes147-1 and 147-2 may be formed via an etching process, and the variableresistance patterns 149 a-1 and 149 a-2 may be formed via a damasceneprocess. In addition, the inner spacers 152-1 and 152-2 may be formed onthe side surfaces of the lower electrodes 141-1 and 141-2 and theselection devices 143-1 and 143-2. In an exemplary embodiment of theinventive concept, the inner spacers 152-1 and 152-2 may be omitted.

In the memory device 100 i according to the present embodiment device,the upper spacers 155 a-1 and 155 a-2 may be formed on the side surfacesof the variable resistance patterns 149 a-1 and 149 a-2. However, sincethe variable resistance patterns 149 a-1 and 149 a-2 have the “L”-shapedstructure, the upper spacers 155 a-1 and 155 a-2 may be formed to havean asymmetric structure. In the case where the damascene process is usedto form the variable resistance patterns 149 a-1 and 149 a-2 with the“L”-shaped structure, an insulating layer may be formed on the heatingelectrodes 147-1 and 147-2, and then, trenches may be formed in theinsulating layer. Here, the trench may be formed to be overlapped withadjacent cells from among the memory cells 140 f-1 and 140 f-2.Thereafter, a first layer, which is to be used as the variableresistance pattern, may be thinly formed on an inner surface of thetrench and on the insulating layer, and then, a second layer, which isto be used as the upper spacer may be formed on the first layer. Next, aplanarization process (e.g., chemical mechanical polishing (CMP)) may beperformed to expose a top surface of the insulating layer. After theplanarization process, mask patterns may be formed to be aligned withthe memory cells 140 f-1 and 140 f-2, and then, the first and secondlayers may be etched using the mask patterns. As a result, the variableresistance patterns 149 a-1 and 149 a-2 having the “L”-shaped structureand the upper spacers 155 a-1 and 155 a-2 may be formed.

In the memory device 100 i according to the present embodiment, thethicknesses of the upper spacers 155 a-1 and 155 a-2 may be adjusted toenhance the resistance characteristics of the variable resistancepatterns 149 a-1 and 149 a-2. For example, the thickness of the upperspacer 155 a-1 or the upper spacer 155 a-2 may be adjusted so that thefirst and second memory cells 140 f-1 and 140 f-2 may have substantiallythe same resistance. Alternatively, or in addition, at least one of thefirst spacer 155 a-1 and the second spacer 155 a-2 may independentlycomprise one of a material exerting a compressive stress and a materialexerting a tensile stress on the corresponding variable resistancepattern 149 a-1 and 149 a-2, and the compressive or tensile stressproperty of the first spacer 155 a-1 or the second spacer 155 a-2 may beadjusted so that the first and second memory cells 140 f-1 and 140 f-2may have substantially the same resistance. Accordingly, the memorydevice 100 i may have enhanced reliability. Also, the thicknesses of theinner spacers 152-1 and 152-2 may be adjusted to enhance the currentcharacteristics of the selection devices 143-1 and 143-2.

Referring to FIG. 15B, a memory device 100 j according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 i of FIG. 15A, in that the memory device 100 j may includevariable resistance patterns 149 b-1 and 149 b-2 that are formed to havea dash (“-”) structure. The dash-shaped (“-”) structure of the variableresistance patterns 149 b-1 and 149 b-2 may be formed using a similarmethod to that for the “L”-shaped structure. For example, a first layer,which is to be used as the variable resistance pattern, may be thinlyformed on an inner surface of the trench and on the insulating layer,and then, an anisotropic etching process may be performed to have aportion of the first layer remain on a side surface of the trench.Thereafter, a second layer may be formed to cover the remaining portionof the first layer. Next, a planarization process (e.g., chemicalmechanical polishing (CMP)) may be performed to expose a top surface ofthe insulating layer. After the planarization process, mask patterns maybe formed to be aligned with the memory cells 140 f-1 and 140 f-2, andthen, the second layer may be etched using the mask patterns. As aresult, the variable resistance patterns 149 b-1 and 149 b-2 having thedash (“-”) structure and the upper spacers 155 a-1 and 155 a-2 may beformed.

To distinguish other structures of the variable resistance pattern fromthe “L”-shaped structure and the dash-shaped (“-”) structure of FIGS.15A and 15B respectively, the structures of the variable resistancepatterns according to the previous embodiments (e.g., of FIGS. 1 to 14)will be referred to as a ‘pillar structure’, and the variable resistancepattern with a large side surface slope will be referred to as a‘pyramid structure’. Meanwhile, this classification may be applied tonot only the variable resistance pattern but also to the selectiondevice; that is, the selection device may be formed to have one of thepillar structure, the “L”-shaped structure, the pyramid structure, andthe dash-shaped (“-”) structure. In the present embodiment, thethickness of the upper spacer 155 a-1 or the upper spacer 155 a-2 may beadjusted so that the first and second memory cells 140 f-1 and 140 f-2may have substantially the same resistance. Alternatively, or inaddition, at least one of the first spacer 155 a-1 and the second spacer155 a-2 may independently comprise one of a material exerting acompressive stress and a material exerting a tensile stress on thecorresponding variable resistance pattern 149 b-1 and 149 b-2, and thecompressive or tensile stress property of the first spacer 155 a-1 orthe second spacer 155 a-2 may be adjusted so that the first and secondmemory cells 140 f-1 and 140 f-2 may have substantially the sameresistance. Accordingly, the memory device 100 j may have enhancedreliability.

Until now, various structures of the memory device have been described.However, the inventive concept is not limited thereto. For example, theinventive concept may be applied to realize any 3D cross-point stackstructure of a memory device, in which spacers having differentthicknesses are formed on side surfaces of memory cells that arepositioned at different levels.

FIG. 16 is a perspective view of a memory device according to anexemplary embodiment of the inventive concept, and FIG. 17 is asectional view taken along lines 2X-2X′ and 2Y-2Y′ of FIG. 16. In thedescription that follows, descriptions of features identical to those ofFIGS. 2 to 3 will be kept to a minimum or omitted in order to avoidredundancy.

Referring to FIGS. 16 and 17, a memory device 1000 according to anexemplary embodiment of the invent concept may include four memory celllayers MCL1, MCL2, MCL3, and MCL4, which are stacked on the substrate101, thereby having a four-layer structure. For example, the firstmemory cell layer MCL1 may be provided between the first electrode linelayer 110L and the second electrode line layer 120L, and the secondmemory cell layer MCL2 may be provided between the second and thirdelectrode line layers 120L and 130L. A second interlayered insulatinglayer 170 may be formed on the third electrode line layer 130L, and afirst upper electrode line layer 210L, a second upper electrode linelayer 220L, and a third upper electrode line layer 230L may be providedon the second interlayered insulating layer 170. The first upperelectrode line layer 210L may include first upper electrode lines 210having a structure substantially the same as that of the first electrodelines 110, the second upper electrode line layer 220L may include secondupper electrode lines 220 having a structure substantially the same asthat of the second electrode lines 120, the third upper electrode linelayer 230L may include third upper electrode lines 230 having astructure substantially the same as that of the first or third electrodelines 110 or 130. A first upper memory cell layer MCL3 may be providedbetween the first and second upper electrode line layers 210L and 220L,and a second upper memory cell layer MCL4 may be provided between thesecond and third upper electrode line layers 220L and 230L.

The first, second, and third electrode line layers 110L, 120L, and 130Land the first and second memory cell layers MCL1 and MCL2 may havesubstantially the same features as those described with reference toFIGS. 2 and 3. The first, second, and third upper electrode line layers210L, 220L, and 230L and the first and second upper memory cell layersMCL3 and MCL4 may also have features substantially the same as those ofthe first, second, and third electrode line layers 110L, 120L, and 130Land the first and second memory cell layers MCL1 and MCL2, except thatthey are provided on the second interlayered insulating layer 170, noton the first interlayered insulating layer 105. Thus, a detaileddescription of each element will be omitted.

Since the third upper electrode lines 230 of the third upper electrodeline layer 230L and the first upper electrode lines 210 of the firstupper electrode line layer 210L may include a structure substantiallythe same as that of the first electrode lines 110, the third upperelectrode line layer 230L may be viewed as the first upper electrodeline layer. Thus, the memory device 1000 may be viewed as to have twofirst upper electrode line layers and one second upper electrode linelayer, and each of the first and second upper memory cell layers MCL3and MCL4 may be positioned between one of the two first upper electrodeline layers and the second upper electrode line layer. For a memorydevice including 6 memory cell layers, a third interlayer insulatinglayer may be provided on the above described structure, and thenadditional first, second, and first upper electrode line layers may besequentially stacked on the third interlayer insulating layer. Each ofthe two additional upper memory cell layers may be positioned betweenone of the two additional first upper electrode line layers and theadditional second upper electrode line layer.

The memory device 1000 according to the present embodiment may have astructure, which may be realized by additionally forming the secondinterlayered insulating layer 170 and the double-layer structure, whichis positioned on the substrate 101 of the memory device 100 shown inFIGS. 2 and 3. However, the structure of the memory device 1000according to the present embodiment is not limited thereto. For example,the memory device 1000 according to the present embodiment may have astructure, which may be realized by additionally forming thedouble-layer structure, which is positioned on the substrate 101 of eachof the memory devices 100 a-100 j of FIGS. 7 to 14, 15A, and 15B, alongwith the second interlayered insulating layer 170. In an exemplaryembodiment of the inventive concept, the memory device 1000 according tothe present embodiment may be configured in such a way that the memorydevice 100 of FIG. 3 is provided below the second interlayeredinsulating layer 170 and the memory device 100 a of FIG. 7 is providedon the second interlayered insulating layer 170, thereby having a hybridstructure. However, structures provided on or below the secondinterlayered insulating layer 170 may be configured to have the samedouble-layer structure, and this may reduce a vertical variation inelectrical characteristics of memory cells.

The memory device 1000 according to the present embodiment may beconfigured to include the four memory cell layers MCL1, MCL2, MCL3, andMCL4 (i.e., to have the four-layer structure), but the inventive conceptis not limited thereto. For example, the inventive concept may beapplied to realize any 3D cross-point stack structure of a memory devicethat is formed by stacking the double-layer structure at least threetimes, along with an interlayered insulating layer. Here, in such a 3Dmemory device, side surfaces of the memory cells may be covered withupper-level spacers, which are provided on common electrode lines (e.g.,the second electrode lines 120), and lower-level spacers, which areprovided under the common electrode lines and have a thickness differentfrom the upper-level spacers.

FIG. 18 is a perspective view of a memory device according to anexemplary embodiment of the inventive concept, and FIG. 19 is asectional view taken along lines 3X-3X′ and 3Y-3Y′ of FIG. 18. In thedescription that follows, descriptions of features identical to those ofFIGS. 2 to 3 will be kept to a minimum or omitted in order to avoidredundancy.

Referring to FIGS. 18 and 19, a memory device 100 k according to anexemplary embodiment of the inventive concept may be similar to thememory device 100 of FIGS. 2 and 3, in that the memory device 100 k maybe configured to include two memory cell layers MCL1 and MCL2 (i.e., tohave a double-layer structure). However, in the memory device 100 kaccording to the present embodiment, first spacers 150T-1 of the firstmemory cells 140-1 and second spacers 150C-2 of the second memory cells140-2 may be provided to have substantially the same thickness but tohave material properties different from each other. For example, thefirst spacer 150T-1 may have a tensile stress property, and the secondspacer 150C-2 may have a compressive stress property. In other words,the first spacer 150T-1 may be configured to exert the tensile stress onthe first memory cell 140-1 (in particular, to the variable resistancepattern 149-1) enclosed therewith, and the second spacer 150C-2 may beconfigured to exert the compressive stress on the second memory cells140-2 (in particular, the variable resistance pattern 149-2) enclosedtherewith.

According to experimental results, when the compressive stress wasexerted on the first and second memory cells 140-1 and 140-2, there wasan increase in resistance (e.g., set resistance) thereof, whereas whenthe tensile stress was exerted, there was a reduction in resistance(e.g., set resistance). Accordingly, by forming a spacer with thetensile stress property on a memory cell of high resistance, resistanceof the memory cell may be reduced. Furthermore, by forming a spacer withthe compressive stress property on a memory cell of low resistance, avertical variation in electrical characteristics of memory cells thatare provided at different levels or layers may be reduced.

In the case where spacers with the same thickness and the same materialproperty are formed on the side surfaces of the first and second memorycells 140-1 and 140-2, resistance of the first memory cells 140-1 may behigher than those of the second memory cells 140-2. In such a case, byproviding the first spacer 150T-1 with the tensile stress property onthe side surface of the first memory cell 140-1, the set resistance ofthe first memory cell 140-1 may be reduced. Furthermore, by providingthe second spacer 150C-2 with the compressive stress property on theside surface of the second memory cell 140-2, the set resistance of thesecond memory cell 140-2 may increase. That is, in the case where thematerial property of the spacer is adjusted in consideration ofresistance characteristics of the first and second memory cells 140-1and 140-2, the resistance characteristics of the first and second memorycells 140-1 and 140-2 may be enhanced.

In the memory device 100 k according to the present embodiment, thefirst spacers 150T-1 with the tensile stress property may be formed onthe side surfaces of the first memory cells 140-1, and the secondspacers 150C-2 with the compressive stress property may be formed on theside surfaces of the second memory cells 140-2. Accordingly, it mayenhance resistance characteristics of the first and second memory cells140-1 and 140-2, and consequently, may reduce a vertical variation inelectrical characteristics of memory cells provided at different levelsor layers. Of course, depending on the resistance characteristics of thefirst and second memory cells 140-1 and 140-2, the first spacer 150T-1with the tensile stress property may be formed on the side surfaces ofthe second memory cells 140-2, and the second spacer 150C-2 with thecompressive stress property may be formed on the side surfaces of thefirst memory cells 140-1.

FIG. 20 is a sectional view of a memory device according to an exemplaryembodiment of the inventive concept and corresponding to the sectionalview of FIG. 19. In the description that follows, descriptions offeatures identical to those of FIGS. 2, 3, 16, 17, 18, and 19 will bekept to a minimum or omitted in order to avoid redundancy.

Referring to FIG. 20, a memory device 100 l according to an exemplaryembodiment of the inventive concept may be different from the memorydevice 100 k of FIG. 19, in that the memory device 100 l may includesecond spacers 150Ca-2 formed to have a different thickness from that ofthe first spacer 150T-1. For example, in the memory device 100 laccording to the present embodiment, the first spacers 150T-1 of thefirst memory cells 140-1 may have a tensile stress property, and thesecond spacers 150Ca-2 of the second memory cells 140-2 may have acompressive stress property. In addition, as shown in FIG. 20, the firstspacer 150T-1 may be formed to have the first thickness T1 greater thana thickness (e.g., the second thickness T2) of the second spacer150Ca-2.

Since the first spacer 150T-1 with the tensile stress property isthickly formed, the resistance of the first memory cells 140-1 mayeffectively decrease. For example, in certain cases, there may be alarge difference in resistance between the first and second memory cells140-1 and 140-2, and it may be necessary to form a spacer protecting thefirst and second memory cells 140-1 and 140-2. In such cases, by thicklyforming the first spacers 150T-1 with the tensile stress property on theside walls of the first memory cells 140-1 having high resistance andforming the second spacer 150Ca-2 with the compressive stress propertyon the side walls of the second memory cells 140-2, the variation inresistance characteristics between the first and second memory cells140-1 and 140-2 may be reduced or minimized.

In the memory device 100 l according to the present embodiment, byadjusting a material property and a thickness of a spacer, resistancecharacteristics of the first and second memory cells 140-1 and 140-2 maybe more precisely controlled. Accordingly, in the memory device 100 laccording to the present embodiment, a vertical variation in resistanceof the first and second memory cells 140-1 and 140-2 may be reduced orminimized.

As described above, in the memory device 100 l according to the presentembodiment, the spacers with different material properties and differentthicknesses may be applied to the memory cells located at differentlevels or layers, but the inventive concept is not limited thereto. Forexample, the spacers of the memory devices 100 a-100 j of FIGS. 7 to 15Bmay also be configured to have at least two different materialproperties. Furthermore, the technical concept is not limited to theabove-described structures of the memory devices. For example, theinventive concept may be applied to realize any memory device, which isconfigured to have a 3D cross-point stack structure, and in which thespacers with different material properties are used for the memory cells140-1 and 140-2 at different levels or layers.

FIG. 21 is a sectional view of a memory device according to an exemplaryembodiment of the inventive concept and corresponding to the sectionalview of FIG. 17. In the description that follows, descriptions offeatures identical to those of FIGS. 2, 3, 16, 17, 18, 19, and 20 willbe kept to a minimum or omitted in order to avoid redundancy.

Referring to FIG. 21, a memory device 1000 a according to an exemplaryembodiment of the inventive concept may be similar to the memory device1000 of FIG. 17, in that the memory device 1000 a may be provided tohave a four-layer structure. However, the memory device 1000 a accordingto the present embodiment may be different from the memory device 1000of FIG. 17, in that the memory device 1000 a may include memory cells140-1, 140-2, 240-1, and 240-2 covered with spacers having differentmaterial properties. For example, in the memory device 1000 a accordingto the present embodiment, the spacers 150T-1 and 250T-1 with thetensile stress property may be formed on the side surfaces of the firstmemory cells 140-1 and the first upper memory cells 240-1, and thespacers 150Ca-2 and 250Ca-2 with the compressive stress property may beformed on the side surfaces of the second memory cells 140-2 and thesecond upper memory cells 240-2. Here, the spacers 150T-1 and 250T-1with the tensile stress property may be formed to a first thickness T1,and the spacers 150Ca-2 and 250Ca-2 with the compressive stress propertymay be formed to have a second thickness T2 that is smaller than thefirst thickness T1.

The memory device 1000 a according to the present embodiment may have afour-layer structure, which may be realized by additionally stacking anupper structure, which is positioned on the substrate 101 of the memorydevice 100 l of FIG. 20, along with the second interlayered insulatinglayer 170, on the memory device 100 l. However, the structure of thememory device 1000 a is not limited thereto. For example, the four-layerstructure of the memory device may be realized by additionally providingan upper structure, which is positioned on the substrate 101 of thememory device 100 k of FIG. 19, along with the second interlayeredinsulating layer 170, on the memory device 100 k. Also, the four-layerstructure of the memory device may be realized by additionally providingan upper structure, which is positioned on the substrate 101 of each ofthe memory devices 100 a-100 j of FIGS. 7 to 14, 15A, and 15B, and inwhich the spacers with different material properties are provided, alongwith the second interlayered insulating layer 170, on each of the memorydevices 100 a-100 j. In addition, the thicknesses and materialproperties of the spacers of the memory devices 100 a-100 j of FIGS. 7to 14, 15A, and 15B having four-layer structure may be properly providedand adjusted, so that the variation in resistance characteristics amongthese four memory cells may be reduced or minimized, and may result inthese four memory cells having substantially the same resistance.

The inventive concept is not limited to the afore-described structuresof the memory device. For example, the inventive concept may be appliedto realize any 3D cross-point stack structure of a memory device that isformed by stacking the double-layer structure at least three times,along with an interlayered insulating layer. Here, in such a 3D memorydevice, side surfaces of the memory cells may be covered withupper-level spacers, which are provided on common electrode lines (e.g.,the second electrode lines 120), and lower-level spacers, which areprovided under the common electrode lines and have a material propertydifferent from the upper-level spacers.

FIGS. 22A to 22L are sectional views for describing a process offabricating a memory device (e.g., of FIG. 3) according to an exemplaryembodiment of the inventive concept. In the description that follows,descriptions of features identical to those of FIGS. 1 to 21 will bekept to a minimum or omitted in order to avoid redundancy.

Referring to FIG. 22A, the interlayered insulating layer 105 may beformed on the substrate 101. The interlayered insulating layer 105 maybe formed of or include, for example, silicon oxide or silicon nitride.However, the material for the interlayered insulating layer 105 is notlimited thereto. Thereafter, the first electrode line layer 110L may beformed on the interlayered insulating layer 105, and the first electrodeline layer 110L may include a plurality of the first electrode lines110, which extend in the first direction X and are spaced apart fromeach other in the second direction Y. The formation of the firstelectrode lines 110 may include a patterning step using an etchingprocess or a damascene process. The first electrode lines 110 may beformed of the same material as that described with reference to FIGS. 2and 3. The first insulating layer 160 a may be formed between the firstelectrode lines 110 to extend in the first direction X.

A lower electrode layer 1411-1, a selection device layer 1431-1, asecond intermediate electrode layer 1451-1, a heating electrode layer1471-1, and a variable resistance pattern layer 1491-1 may besequentially stacked on the first electrode line layer 110L and thefirst insulating layer 160 a to form a first stack 1401-1. Each of thelayers constituting the first stack 1401-1 may be the same as thecorresponding one in FIGS. 2 and 3 in terms of its material or function.

Referring to FIG. 22B, after the formation of the first stack 1401-1,mask patterns, which are spaced apart from each other in the first andsecond directions X and Y, may be formed on the first stack 1401-1.Next, the first stack 1401-1 may be etched using the mask patterns as anetch mask to partially expose top surfaces of the first insulating layer160 a and the first electrode lines 110. As a result, a plurality of thefirst memory cells 140-1 may be formed on the substrate 101.

The first memory cells 140-1 may be formed to have the same arrangementas the mask patterns. That is, the first memory cells 140-1 may bespaced apart from each other in the first and second directions X and Y.Furthermore, the first memory cells 140-1 may be electrically connectedto the first electrode lines 110 thereunder. Each of the first memorycells 140-1 may include the lower electrode 141-1, the selection device143-1, the second intermediate electrode 145-1, the heating electrode147-1, and the variable resistance pattern 149-1 sequentially stacked onthe first electrode line 110. After the formation of the memory cells140-1, the mask patterns may be removed via an ashing and/or stripprocess.

Referring to FIG. 22C, an inner spacer layer 1521-1 may be formed to auniform thickness on the first memory cells 140-1, the first insulatinglayer 160 a, and the first electrode lines 110. The inner spacer layer1521-1 may be formed by using a suitable conformal deposition technique(e.g., CVD or ALD). The inner spacer layer 1521-1 may be the same as theinner spacers 152-1 and 152-2 of FIGS. 2 and 3 in terms of its materialor function.

Referring to FIG. 22D, the inner spacer layer 1521-1 may be etchedusing, for example, an etch-back process and/or a dry etching process tohave portions of the inner spacer layer 1521-1 remain on the sidesurfaces of the lower electrode 141-1 and the selection device 143-1,and to remove the other portions from other regions. The portions of theinner spacer layer 1521-1 remaining on the side surfaces of the lowerelectrode 141-1 and the selection device 143-1 may constitute the firstinner spacers 152-1.

Referring to FIG. 22E, a first spacer layer 1501-1 may be formed to auniform thickness on the first memory cells 140-1, the first insulatinglayer 160 a, the first electrode lines 110, and the first inner spacers152-1. The first spacer layer 1501-1 may be formed by using a suitableconformal deposition technique (e.g., CVD or ALD) and may besubstantially the same as the spacers 150-1 and 150-2 of FIGS. 2 and 3in terms of its material or function.

The first spacer layer 1501-1 may be formed to have a first initialthickness T1′. The first initial thickness T1′ may be suitablydetermined in consideration of a first thickness T1 of the first spacer150-1, which is to be determined as a result of a subsequent etchingprocess.

Referring to FIG. 22F, the first spacer layer 1501-1 may be etchedusing, for example, an etch-back process and/or a dry etching process,and thus, the first spacers 150-1 may be formed on the side surfaces ofthe first memory cells 140-1. As described above, the first spacer 150-1may be formed to have the first thickness T1 and to cover the firstinner spacer 152-1.

Referring to FIG. 22G, the second insulating layer 160 b may be formedto fill a space between the first memory cells 140-1. The secondinsulating layer 160 b may be formed of an insulating material (e.g.,oxide or nitride) that may be the same as or different from that of thefirst insulating layer 160 a. The insulating layer may be formed to athickness that is large enough to completely fill gap regions betweenthe first memory cells 140-1, and then a chemical-mechanical polishing(CMP) process may be performed to expose the top surface of the variableresistance pattern 149-1 and thereby to form the second insulating layer160 b.

The second electrode lines 120 may be formed by forming a conductivelayer for the second electrode line layer and patterning the conductivelayer using an etching process. The second electrode lines 120 mayextend in the second direction Y and may be spaced apart from each otherin the first direction X. The third insulating layer 160 c extending inthe second direction Y may be provided between the second electrodelines 120.

As described above, the second electrode lines 120 may be formed via anetching process, but the inventive concept is not limited thereto. Forexample, the second electrode lines 120 may be formed via a damasceneprocess. The damascene process for forming the second electrode lines120 may include forming an insulating layer on the first memory cells140-1 and the second insulating layer 160 b and etching the insulatinglayer to form trenches extending in the second direction Y and exposingthe top surface of the variable resistance pattern 149-1. Thereafter, aconductive material may be formed to fill the trenches, and the secondelectrode lines 120 may be formed by planarizing the conductivematerial. In an exemplary embodiment of the inventive concept, theinsulating layer filling the gap regions between the first memory cells140-1 may be formed to be thick enough to form the trenches therein, andthe second electrode lines 120 may be formed in the trench. In thiscase, the second and third insulating layers 160 b and 160 c may beformed of the same material and may be connected to each other, therebyhaving a single-body structure.

Referring to FIG. 22H, a second stack, which has the same structure asthat of the first stack 1401-1 of FIG. 22A, may be formed on the secondelectrode lines 120. The second stack may be patterned to form thesecond memory cells 140-2, which are spaced apart from each other in thefirst and second directions X and Y and are electrically connected tothe second electrode lines 120. Similar to the first memory cells 140-1,each of the second memory cells 140-2 may include the lower electrode141-2, the selection device 143-2, the second intermediate electrode145-2, the heating electrode 147-2, and the variable resistance pattern149-2 sequentially stacked on the second electrode line 120.

Referring to FIG. 22I, the second inner spacers 152-2 may be formed onthe side surfaces of the second memory cells 140-2. The second innerspacers 152-2 may be formed by using the same method as that for thefirst inner spacers 152-1 described with reference to FIGS. 22C and 22D.Each of the second inner spacers 152-2 may be formed to cover sidesurfaces of the lower electrode 141-2 and the selection device 143-2.

Referring to FIG. 22J, a second spacer layer 1501-2 may be formed to auniform thickness on the second memory cells 140-2, the third insulatinglayer 160 c, the second electrode lines 120, and the second innerspacers 152-2. The second spacer layer 1501-2 may be formed by using asuitable conformal deposition technique (e.g., CVD or ALD) and may besubstantially the same as the spacers 150-1 and 150-2 of FIGS. 2 and 3in terms of its material or function.

The second spacer layer 1501-2 may be formed to have an initial secondthickness T2′. The initial second thickness T2′ may be suitablydetermined in consideration of a second thickness T2 of the secondspacer 150-2, which is to be determined as a result of a subsequentetching process.

Referring to FIG. 22K, the second spacer layer 1501-2 may be etchedusing, for example, an etch-back process and/or a dry etching process toform the second spacers 150-2 on the side surfaces of the second memorycells 140-2. As described above, the second spacer 150-2 may have thesecond thickness T2. As shown in FIG. 22K, the second spacer 150-2 maynot cover the second inner spacer 152-2. The second spacer 150-2 mayonly cover the side surfaces of the second intermediate electrode 145-2,the heating electrode 147-2, and the variable resistance pattern 149-2.However, in an exemplary embodiment of the inventive concept, the secondspacer 150-2 may be formed to cover the second inner spacer 152-2.

Referring to FIG. 22L, the fourth insulating layer 160 d may be formedto fill gap regions between the second memory cells 140-2, and then, thethird electrode lines 130 may be formed on the second memory cells 140-2and the fourth insulating layer 160 d. The resulting structure providedwith the third electrode lines 130 may be the same as that of the memorydevice 100 of FIG. 3. The third electrode lines 130 may be formed byusing a similar method to that for the second electrode lines 120described with reference to FIG. 22G. However, like the first electrodelines 110, the third electrode lines 130 may extend in the firstdirection X and may be spaced apart from each other in the seconddirection Y, and the fifth insulating layer 160 e extending the firstdirection X may be provided between the third electrode lines 130. In anexemplary embodiment of the inventive concept, the fourth insulatinglayer 160 d and the fifth insulating layer 160 e may be formed to have asingle-body structure.

Thereafter, the second interlayered insulating layer 170 may be formedon the third electrode lines 130, and the process steps of FIGS. 22A to22L may be performed again on the second interlayered insulating layer170 to realize the memory device 1000 of the four-layer structuredescribed with reference to FIG. 17. Furthermore, the process steps maybe further performed to realize a memory device having a six-or-morelayer structure.

FIGS. 23A to 23C are sectional views for describing a process offabricating a memory device (e.g., of FIG. 3) through a method differentfrom that for the first memory cells 140-1 of FIG. 22B.

Referring to FIG. 23A, a patterning process using a line-shaped firstmask pattern extending in the first direction X may be performed on thefirst stack 1401-1 (e.g., of FIG. 22A) to form a plurality of first linestructures 140 x-1, which extend in the first direction X and are spacedapart from each other in the second direction Y.

Referring to FIG. 23B, an insulating layer may be formed to fill gapregions between the first line structures 140 x-1 and may be planarizedto expose a top surface of a variable resistance pattern 149 x-1. Afirst gap-fill layer 190 may be formed as a result of the planarizationof the insulating layer.

Referring to FIG. 23C, a line-shaped second mask pattern extending inthe second direction Y may be formed on the first line structures 140x-1 and the first gap-fill layer 190. The first line structures 140 x-1and the first gap-fill layer 190 may be etched using the second maskpattern to form a plurality of first memory cells 140-1 (e.g., of FIG.22B) spaced apart from each other in the first and second directions Xand Y. Thereafter, a remaining portion 190 of the first gap-fill layermay be removed to form the first memory cells 140-1 shown in FIG. 22B.

FIG. 24 is a block diagram of a computer system according to anexemplary embodiment of the inventive concept.

Referring to FIG. 24, a computer system 1200 may include a processor1220 and a memory system 1210. The processor 1220 may include aplurality of cores, which are configured to execute commands and processdata, and one or more processor caches, which are configured to storethe commands and data. In addition, the processor may further include amemory controller configured to control the one or more caches and amemory device, which is provided in the memory system 1210. For example,the processor 1220 may include at least one of a memory-side cache (MSC)controller, a nonvolatile RAM controller, and an integrated memorycontroller. Meanwhile, the processor 1220 may include an I/O sub system,and the processor 1220 may communicate an external network and/or I/Odevices through the I/O sub system.

The memory system 1210 may include a first memory device 1210-1 and asecond memory device 1210-2. The first and second memory devices 1210-1and 1210-2 may be classified based on which memory channels are used toconnect them to the processor 1220. Thus, the memory channel may includeat least one signal line connected to the processor. The first memorydevice 1210-1 may be connected to the processor 1220 through a firstmemory channel CH1. The first memory device 1210-1 may include two typesof memory devices. For example, the first memory device 1210-1 mayinclude a first level memory 1202-1 and a second level memory 1204-1.The first level memory 1202-1 may have a first operation speed (e.g., afirst read access and a first write access speed). Also, the secondlevel memory 1204-1 may have a second operation speed (e.g., a secondread access speed and a second write access speed). Here, the firstoperation speed may be faster than the second operation speed.Meanwhile, the first level memory 1202-1 with a high operation speed maybe used as a cache for temporarily storing commands or data to be storedin the second level memory 1204-1.

The second memory device 1210-2 may be connected to the processor 1220through a second memory channel CH2. The second memory device 1210-2 mayalso include two types of memory devices. For example, the second memorydevice 1210-2 may include a first level memory 1202-2 and a second levelmemory 1204-2. The first level memory 1202-2 may have the firstoperation speed, and the second level memory 1204-2 may have the secondoperation speed. In the second memory device 1210-2, the first levelmemory 1202-2 with a high operation speed may be used as a cache fortemporarily storing commands or data to be stored in the second levelmemory 1204-2.

Each of the first level memories 1202-1 and 1202-2 may include, forexample, a DRAM device. Each of the second level memories 1204-1 and1204-2 may include, for example, a nonvolatile RAM device. Here, thenonvolatile RAM device may include one of phase-change RAM (PRAM),ReRAM, and MRAM devices. In an exemplary embodiment of the inventiveconcept, the nonvolatile RAM may include at least one of the memorydevice 100 shown in FIGS. 1 to 3, the memory devices 100 a-100 j shownin FIGS. 7 to 14, 15A, and 15B, the memory device 1000 shown in FIGS. 16and 17, the memory device 100 k shown in FIGS. 18 and 19, the memorydevice 100 l shown in FIG. 20, and the memory device 1000 a shown inFIG. 21.

While the inventive concept has been particularly shown and describedwith reference to the exemplary embodiments of the inventive conceptthereof, it will be understood that various changes in form and detailsmay be made therein without departing from the spirit and scope of thefollowing claims.

What is claimed is:
 1. A memory device, comprising: a first electrodeline layer provided on a substrate, the first electrode line layercomprising a plurality of first electrode lines which extend in a firstdirection and are spaced apart from each other; a second electrode linelayer provided on the first electrode line layer, the second electrodeline layer comprising a plurality of second electrode lines which extendin a second direction different from the first direction and are spacedapart from each other; a third electrode line layer provided on thesecond electrode line layer, the third electrode line layer comprising aplurality of first electrode lines; a first memory cell layer providedbetween the first and second electrode line layers, the first memorycell layer comprising a plurality of first memory cells arranged atrespective intersections of the plurality of first electrode lines ofthe first electrode line layer and the plurality of second electrodelines; a second memory cell layer provided between the second and thirdelectrode line layers, the second memory cell layer comprising aplurality of second memory cells arranged at respective intersections ofthe plurality of second electrode lines and the plurality of firstelectrode lines of the third electrode line layer; a first spacercovering side surfaces of each of the plurality of first memory cells;and a second spacer covering side surfaces of each of the plurality ofsecond memory cells, wherein each of the plurality of first and secondmemory cells comprises a selection device, an electrode, and a variableresistance pattern stacked in an upward or downward direction, and thefirst spacer has a thickness different from that of the second spacer.2. The memory device of claim 1, wherein the thickness of each of thefirst and second spacers is a thickness measured in a direction normalto a side surface of the variable resistance pattern, and the thicknessof the first or second spacer is adjusted so that the first and secondmemory cells have substantially the same resistance.
 3. The memorydevice of claim 2, wherein the resistance is a set or reset resistanceof the first and second memory cells.
 4. The memory device of claim 3,wherein the set resistance of the first or second memory cells coveredby the first or second spacer having a large thickness is substantiallythe same as the set resistance of the second memory cells or the firstmemory cells covered by the second spacer or the first spacer having asmall thickness.
 5. The memory device of claim 1, wherein each of thefirst and second memory cells comprises a heating electrode in contactwith the variable resistance pattern.
 6. The memory device of claim 1,wherein each of the first and second memory cells comprises a lowerelectrode in contact with the selection device, and side surfaces of theselection device and the lower electrode are covered by an inner spacer.7. The memory device of claim 1, wherein one of the first and secondspacers comprises a material exerting a compressive stress on thevariable resistance pattern, and the other of the first and secondspacers comprises a material exerting a tensile stress on the variableresistance pattern.
 8. The memory device of claim 1, further comprising:at least one first upper electrode line layer provided on the thirdelectrode line layer, each of the at least one first upper electrodeline layer comprising a plurality of first electrode lines; at least onesecond upper electrode line layer provided on a corresponding one of theat least one first upper electrode line layer, each of the at least onesecond upper electrode line layer comprising a plurality of secondelectrode lines; and at least two upper memory cell layers providedbetween the first upper electrode line layer and the second upperelectrode line layer, each of the at least two upper memory cell layerscomprising a plurality of memory cells arranged at respectiveintersections of the plurality of first electrode lines of the firstupper electrode line layer and the plurality of second electrode linesof the second upper electrode line layer.
 9. The memory device of claim1, wherein the variable resistance pattern comprises at least one ofGeSbTe, InSbTe, and BiSbTe, or comprises a super lattice structure, inwhich GeTe and SbTe layers are alternately stacked.
 10. The memorydevice of claim 1, wherein each of the first and second memory cellscomprises a heating electrode in contact with the variable resistancepattern, and the heating electrode comprises a carbon-based conductivematerial.
 11. The memory device of claim 1, wherein the selection devicecomprises one of an ovonic threshold switching (OTS) device, a diode,and a transistor.
 12. The memory device of claim 1, wherein at least oneof the selection device and the variable resistance pattern has one of apillar structure, a pyramid structure, an “L”-shaped structure, and adash-shaped structure.
 13. A memory device, comprising: a firstelectrode line layer provided on a substrate, the first electrode linelayer comprising a plurality of first electrode lines which extend in afirst direction and are spaced apart from each other; a second electrodeline layer provided on the first electrode line layer, the secondelectrode line layer comprising a plurality of second electrode lineswhich extend in a second direction different from the first directionand are spaced apart from each other; a third electrode line layerprovided on the second electrode line layer, the third electrode linelayer comprising a plurality of first electrode lines; a first memorycell layer provided between the first and second electrode line layers,the first memory cell layer comprising a plurality of first memory cellsarranged at respective intersections of the plurality of first electrodelines of the first electrode line layer and the plurality of secondelectrode lines; a second memory cell layer provided between the secondand third electrode line layers, the second memory cell layer comprisinga plurality of second memory cells arranged at respective intersectionsof the plurality of second electrode lines and the plurality of firstelectrode lines of the third electrode line layer; a first spacercovering side surfaces of each of the plurality of first memory cells;and a second spacer covering side surfaces of each of the plurality ofsecond memory cells, wherein each of the plurality of first and secondmemory cells comprises a selection device, an electrode, and a variableresistance pattern stacked in an upward or downward direction, and atleast one of the first spacer and the second spacer comprises a materialexerting a compressive or tensile stress on the variable resistancepattern.
 14. The memory device of claim 13, wherein one of the firstspacer and the second spacer comprises a material exerting a compressivestress on the variable resistance pattern, and the other comprises amaterial exerting a tensile stress on the variable resistance pattern.15. The memory device of claim 13, wherein each of the first and secondmemory cells comprises a lower electrode in contact with the selectiondevice, side surfaces of the selection device and the lower electrodeare covered by an inner spacer, and at least one of the first spacer andthe second spacer covers the inner spacer.
 16. The memory device ofclaim 13, further comprising: at least one first upper electrode linelayer provided on the third electrode line layer, each of the at leastone first upper electrode line layer comprising a plurality of firstelectrode lines; at least one second upper electrode line layer providedon a corresponding one of the at least one first upper electrode linelayer, each of the at least one second upper electrode line layercomprising a plurality of second electrode lines; and at least two uppermemory cell layers provided between the first upper electrode line layerand the second upper electrode line layer, each of the at least twoupper memory cell layers comprising a plurality of memory cells arrangedat respective intersections of the plurality of first electrode lines ofthe first upper electrode line layer and the plurality of secondelectrode lines of the second upper electrode line layer.
 17. Anelectronic apparatus, comprising: a processor configured to performcommands and to process data; a memory channel comprising at least onesignal line connected to the processor; a first memory device connectedto the processor through the memory channel, the first memory devicecomprising a first level memory having a first operation speed and anonvolatile property; and a second memory device connected to theprocessor through the memory channel, the second memory devicecomprising a second level memory having a second operation speed that isfaster than the first operation speed, wherein the first level memorycomprises at least two memory cell layers, each of which has across-point structure and comprises a plurality of memory cells, each ofthe plurality of memory cells comprises a selection device, anelectrode, and a variable resistance pattern, and a first spacercovering the side surfaces of each of the plurality of memory cells inone of the at least two memory cell layers has a thickness differentfrom that of a second spacer covering the side surfaces of each of theplurality of memory cells in at least one other of the at least twomemory cell layers.
 18. The electronic apparatus of claim 17, whereinthe cross-point structure comprises: a first electrode line layercomprising a plurality of first electrode lines, which extend in a firstdirection and are spaced apart from each other; a second electrode linelayer provided on or below the first electrode line layer, the secondelectrode line layer comprising a plurality of second electrode lines,which extend in a second direction different from the first directionand are spaced apart from each other; and the plurality of memory cellsarranged between the first electrode line layer and the second electrodeline layer and at respective intersections of the plurality of firstelectrode lines and the plurality of second electrode lines, wherein thefirst memory device comprises the at least two memory cell layers, whichare realized by extending the cross-point structure in a third directionperpendicular to the first and second directions.
 19. The electronicapparatus of claim 17, wherein each of the plurality of memory cellscomprises a lower electrode in contact with the selection device, sidesurfaces of the selection device and the lower electrode are covered byan inner spacer, and the spacers covering the side surfaces of theplurality of memory cells cover the inner spacers.
 20. The electronicapparatus of claim 17, wherein the first spacer covering the sidesurfaces of the plurality of memory cells in the one of the at least twomemory cell layers comprises a material exerting a compressive stress onthe variable resistance pattern, and the second spacer covering the sidesurfaces of the plurality of memory cells in the at least one other ofthe at least two memory cell layers comprises a material exerting atensile stress on the variable resistance pattern.